Frontend for Merge Request Diff

This merge request improves the UX for the merge request diff feature which was recently implemented here (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127). Specifically, it styles various parts of the diff feature to match the designs, it disables comment-related buttons in states where comments are disabled, and it adds a 'Show latest version' button for convenience.

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

I could use feedback on this MR's fidelity to the design.  

## Why was this MR needed?

Neccessary styling improvements for basic UX of this feature, and enabled comment buttons are not functional and thus need to be disabled in certain states.

## Screenshots (if relevant)

![57dd0755f0b14342305909](/uploads/318a44a3bc8b7fc5c9c6ef92ba92e511/57dd0755f0b14342305909.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] 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 you do - 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/21427
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127

cc: @jschatz1 

See merge request !6343