Render HipChat notification descriptions as HTML instead of raw markdown

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/1621 for issues, merge requests and comments.

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

This MR is small enough to double check everything.

Why was this MR needed?

Because seeing raw markdown in HipChat notifications is not nice :)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/1621