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 :)

See merge request !6371