Moved all of reviewboard.utils and reviewboard.reviews.templatetags.emailtags into djblets.util.

The template tags have been split into sections: djblets_utils (which has most of the old htmlutils), djblets_email (which has the old emailtags), djblets_deco (which contains the box tags), and djblets_js (which for now contains only the JavaScript dialog serialization code).

The Review Board side of this change will be in a separate review request.

Made sure Review Board's unit tests test Djblets and that everything works fine. Went to all the main Review Board pages without problems.