New Project page improvements for mobile

## What does this MR do?

Redesigns the "Project path" fields on the New Project page.

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

Not that I know of, nothing I changed should break any other pages.

## Why was this MR needed?

The new project page was unusable on mobile because the project name text field didn't have enough space.

## What are the relevant issue numbers?

Fixes #18599.

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-15_at_12.38.58_PM](/uploads/046a824b283e24266cabc9ea3dfb169c/Screen_Shot_2016-06-15_at_12.38.58_PM.png)

After:

![Screen_Shot_2016-06-15_at_12.37.19_PM](/uploads/6d01310192965b413bfedd9c8d2129f7/Screen_Shot_2016-06-15_at_12.37.19_PM.png)

![Screen_Shot_2016-06-15_at_12.37.28_PM](/uploads/3944a0f0ab04373ec92f7cafe90872df/Screen_Shot_2016-06-15_at_12.37.28_PM.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] 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)

cc: @jschatz1 @dzaporozhets 

See merge request !4687