Put a link to markdown documentation next to rich-text fields.

Appended the markdown link next to the buttons in 'Description', 'Testing
Done', 'Comments' and 'Reply to comments' fields.

Testing Done:
For each view, checked the state of all combinations of buttons
pressed/unpressed to make sure the markdown link appears when beginning edit
and disappears when cancelling edit. For reviewReply and reviewRequest views,
opened multiple text editors at the same time to check if the markdown
informaiton appears.

Bugs closed: 3201

Reviewed at https://reviews.reviewboard.org/r/5378/