Add markdown editing buttons to the Web IDE
We have nice buttons to allow you to edit your markdown easier for most forms in GitLab:

When editing or creating a markdown file in the Web IDE, this should also be available.
### Proposal
When we detect you're working on a markdown file, we should show that bar, which works in exactly the same way.
## Links / references
https://gitlab.com/gitlab-org/gitlab-ce/issues/27861
issue