Make milestone page responsive

Credit to @yury-n for this one. ⭐ 🌟 ⭐  Original MR: !5255

## What does this MR do?

> Makes the milestone page more mobile friendly

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

> Shouldn't be but maybe double check that the views I changed are exclusively used by the milestone details page I was amending and not any other pages that I haven't tested after my changes.

Looks good to me. :) Would be awesome to have @annabeldunstone check up on the stylesheet updates.

## Why was this MR needed?

> Buttons were misaligned

> Progress bar was not 100% width

> Stats were cut in the middle

> Milestone name/date text overflow

![Screen_Shot_2016-07-14_at_5.37.49_PM](/uploads/4da67c9f81d71873188e4a5cafed8113/Screen_Shot_2016-07-14_at_5.37.49_PM.png)

![Screen_Shot_2016-07-14_at_5.41.58_PM](/uploads/b3d767ce49e37b251c3192ed051400b8/Screen_Shot_2016-07-14_at_5.41.58_PM.png)

![Screen_Shot_2016-07-14_at_5.46.41_PM](/uploads/353ad59a88f3dea9e3f88e6ea05c1af3/Screen_Shot_2016-07-14_at_5.46.41_PM.png)

## Screenshots (if relevant)

![Screen_Shot_2016-07-14_at_6.15.58_PM](/uploads/a26768701ea6cb00ccd85a5ac0772e36/Screen_Shot_2016-07-14_at_6.15.58_PM.png)

![Screen_Shot_2016-07-14_at_5.57.04_PM](/uploads/b94cf1b7593589948216342b48a19831/Screen_Shot_2016-07-14_at_5.57.04_PM.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 [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 you do - 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 #18852

See merge request !6655