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/