Apply better hierarchy to markdown headers and issue/mr titles

## What does this MR do?
Applies better hierarchy to markdown headers and issue/mr titles

## Why was this MR needed?
Titles of issues/mr's were smaller than h1, leading to strange hierarchy when reading descriptions

## Screenshots (if relevant)

### Before:
![Screen_Shot_2016-10-14_at_1.31.37_PM](/uploads/79b6b16b8ed1b3b49578161eea1f0e42/Screen_Shot_2016-10-14_at_1.31.37_PM.png)

### After:
![Screen_Shot_2016-10-14_at_1.31.26_PM](/uploads/f710b63c112e342c5429810984079da6/Screen_Shot_2016-10-14_at_1.31.26_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 it does - 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 #21395

See merge request !6904