Sandbox reviewboard.reviews.ui.ReviewUI

Extensions that create a FileAttachmentReviewUI subclass (using the
ReviewUIHook) can throw exceptions inside Review Board. The parts of Review
Board that call those methods have been sandboxed.

Now when a FileAttachmentReviewUI subclass from an extension throws an
exception, Review Board logs the errors with enough information to find which
method in the FileAttachmentReviewUI subclass threw the exception.

Testing Done:
Unit tests have been written to make sure that functions from an
FileAttachmentReviewUI subclass have been called, and when an exception is
thrown it gets logged.

The tests fail without the sanboxing, and succeed with it.

Reviewed at https://reviews.reviewboard.org/r/6551/
Sandbox reviewboard.attachments.mimetypes.MimetypeHandler

Extensions that create a MimetypeHandler subclass can throw exceptions inside
Review Board. The sections of Review Board that call those methods have been
sandboxed.

Now when a MimetypeHandler subclass from an extension throws an exception,
Review Board logs the errors. The error logs give enough information to find
which method in the MimetypeHandler subclass threw the exception.

Testing Done:
Unit tests have been written to make sure that functions from an
MimetypeHandler subclass have been called, and when an exception is thrown it
gets logged.

The tests fail without the sandboxing, and succeed with it.

Reviewed at https://reviews.reviewboard.org/r/6532/
Limit e-mail recipients when adding reviewers.

Occasionally, the only change to a review request will be to add additional
reviewers. This is especially common when a review request is first sent to a
generic group, and then people in that group make suggestions about who might
be more appropriate to review the code in question.

This change makes it so that when the only change to a review request is to
change the list of reviewers (people or groups), the e-mail that is sent is
sent only to the author of the review request (assuming that they have "own
actions" e-mails turned on), people who have starred the review request, and
the newly-added reviewers. This was requested by a user who has some very
high-volume e-mail lists, in an effort to cut down on the amount of traffic.

In the case where the change was to remove reviewers, the e-mail is sent only
to the author. In the case that the author has "own actions" e-mails turned
off, no e-mail will be sent.

Testing done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6683/