To keep HTML class consistent through whole project.
## What does this MR do? This MR fixes New Issue button class on a Milestone to be consistent with ones in other places. ~"UI polish" ~milestones ## Are there points in the code the reviewer needs to double check? No. ## Why was this MR needed? To keep HTML class consistent through whole project. ## Screenshots (if relevant) ![9698-new-issue-btn-on-milestone](/uploads/d92b60e0f26a87426525d8bcc6a0882a/9698-new-issue-btn-on-milestone.png) ## Does this MR meet the acceptance criteria? - [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added - [n/a] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) - [n/a] API support added - Tests - [x] Added for this feature/bug - [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?