- Tab between register and sign in forms
- Add individual input validation error messages
- Validate username
- Update many styles for all login-box forms

Enforce TrailingSemicolon and EmptyLineBetweenBlocks in scss-lint

## What does this MR do?
Enforces two new scss-lint rules
1.  TrailingSemicolon
2.  EmptyLineBetweenBlocks

## Are there points in the code the reviewer needs to double check?
Make sure I didn't break anything 😁 

## Why was this MR needed?
Improve our SCSS, Lessen the burden of catching these during the review process

## Screenshots (if relevant)
NOne

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #23319

See merge request !6903

Added subnav to all views bar issuable views

## What does this MR do?

Adds subnav to all views apart from issue and MR views. The full list is available in screenshots below.

It also adds some JS to the builds page to simply handle the repositioning of the fixed position sidebar with the addition of the subnav.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

~UX and apparently ~regression .

## What are the relevant issue numbers?

Closes #18844.

## Screenshots (if relevant)

### Label

#### Edit

![Screen_Shot_2016-08-24_at_23.45.25](/uploads/3e5ded9e60f0eccc37672ab0a1423bfb/Screen_Shot_2016-08-24_at_23.45.25.png)

#### New

![Screen_Shot_2016-08-24_at_23.45.13](/uploads/767c6e97c91a5ded904f5b495950f5a8/Screen_Shot_2016-08-24_at_23.45.13.png)

### Milestone

#### Edit

![Screen_Shot_2016-08-24_at_23.45.37](/uploads/27562162415a6ae403e7117df009b208/Screen_Shot_2016-08-24_at_23.45.37.png)

#### New

![Screen_Shot_2016-08-24_at_23.45.43](/uploads/43c655b0a2af9a8da335a2efc31cb2df/Screen_Shot_2016-08-24_at_23.45.43.png)

#### Show

![Screen_Shot_2016-08-24_at_23.45.53](/uploads/68c3e0788c81f5897c7db11bb2b02a4c/Screen_Shot_2016-08-24_at_23.45.53.png)

### Commit

#### Show

![Screen_Shot_2016-08-26_at_14.15.20](/uploads/9f96e0587e4866fa95e28c224a185844/Screen_Shot_2016-08-26_at_14.15.20.png)

#### Builds

![Screen_Shot_2016-08-26_at_15.28.46](/uploads/9f0d8ababc8362874a3e2b5d877c7668/Screen_Shot_2016-08-26_at_15.28.46.png)

### Blob

#### Blame

![Screen_Shot_2016-08-26_at_14.18.03](/uploads/2999f2cd304e3b9be6530a52407eca59/Screen_Shot_2016-08-26_at_14.18.03.png)

#### Edit

![Screen_Shot_2016-08-26_at_14.20.32](/uploads/bb7b00bbe835228a8ec8b371e9364cb8/Screen_Shot_2016-08-26_at_14.20.32.png)

### Pipelines

#### Show

![Screen_Shot_2016-08-26_at_15.44.30](/uploads/ee84c51597ab04a07a6c953704280e7b/Screen_Shot_2016-08-26_at_15.44.30.png)

#### Show build

![Screen_Shot_2016-08-26_at_15.45.55](/uploads/dd78ef3dc083186095cdc84f41c80b21/Screen_Shot_2016-08-26_at_15.45.55.png)

#### Edit Envrionment

![Screen_Shot_2016-08-26_at_15.48.33](/uploads/4e9340e11eedeaeddebb708f02db3598/Screen_Shot_2016-08-26_at_15.48.33.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #18844

See merge request !5998

Improved specificity of toggleable file headers

## What does this MR do?

Improves the specificity of the diff file title toggle styling.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

Currently, any `.file-title` will have a pointer cursor and background colour change on hover. This should be only for diff files, except discussion diff files.

## Screenshots (if relevant)

![2016-10-14_13.57.12](/uploads/e8561bed378487b8d14d579ce31417d0/2016-10-14_13.57.12.gif)![2016-10-14_14.01.15](/uploads/e88858874377fa83aa5e632ec9e1a601/2016-10-14_14.01.15.gif)![2016-10-14_14.01.46](/uploads/8a773ee6bb29386f2dc022bb2ffaf474/2016-10-14_14.01.46.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #23147

See merge request !6887