Send email when closing review requests

Optionally send emails when closing review requests. Add a flag in the Email
settings page that will allow for sending emails when a review request is closed
(Submitted/Discarded).

Files Changed:
reviewboard/admin/forms.py
+ Added Boolean to store the value of "mail_send_review_close_mail" in settings

reviewboard/notifications/email.py
+ Added callback method for review_request_closed signal

reviewboard/templates/notifications/review_request_email.html
reviewboard/templates/notifications/review_request_email.txt
+ Added code to display the "Status" of the review request when it's closed
  before the "Description" section.

Tests:
+ Added test for review request closing
+ Modified other review related tests to fix counts or expected results

Testing done: Added test for this feature and ran all tests. Verified in UI.

Fixes bug 422
Reviewed at http://reviews.reviewboard.org/r/3707/