Remove leading and trailing blank lines in review comments.

When a user submits a review comment, leading and trailing
blank lines are saved as part of the comment. We want to strip
these blank lines before saving the comment to the database.

Testing done:
Created new review request and manually tested:

-review
-review reply
-diff comment
-reply diff comment
-file attachment comment
-reply file attachment comment

Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/4561/
Fixes issue 2933