Prevent users from publishing empty (no modifications made) review request drafts.

When a user submits an empty draft for a review request a warning is issued to
let them know they did not make any modifications.

The general flow is:
1. In the review_request_draft model determine if there were any changes to any
   of the fields, if not then raise a 'Not Modified' exception.
2. The review_request_draft controller/resource will catch the 'Not Modified'
   exception and return an appropriate error message to the Client JS to
   display a meaningful message to the user.
3. Client JS displays error message and the 'Publish Changes' and 'Discard
   Draft' buttons re-enable.

Testing done:
Tried "not-modifying" the Description, and Testing Done fields to determine the
output.

Wrote test case to publish an empty draft (test case passes).

Postive Test Cases:
* Added a file (successful publish).
* Remove a file (successful publish).
* Changing the description field (successful publish).
* Add comments (successful publish).
* Add branch/target people/bug number.

Fixes bug 3452.

Reviewed at https://reviews.reviewboard.org/r/6322/