Issue 3189: Add a link to the interdiff from the review request draft banner

Adds a link to the interdiff from the review request draft banner to make it
easier to see the change the draft contains.

Testing Done:
* Tested the change using a development review board with different drafts
  (with and without diffsets).
* Unit tests pass.

Bugs closed: 3189

Reviewed at https://reviews.reviewboard.org/r/5384/