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
## Screenshots (if relevant)
## 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
