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 its closed
    before the "Description" section.
    
* Tests:
  + Added test for review request closing
  + Modified other review related tests to fix counts or expected results

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