Adding 'drop_open_issues' to StatusUpdate.
This feature is needed by the Review Bot feature "Adding ability to drop old issues on new review." https://reviews.reviewboard.org/r/10061/.

The method finds all open issues associated with the status update's review (if it exists) and drops them. Smart enough to update the review request's timestamp and issues counts.

Unit tests!
Also, called the method (via the Review Bot changes) with a review request with previous status updates with open issues. Noticed the timestamp and issue count were correct, and old issues were dropped.