Increase the size of the review request actions.

The review request actions were hard to hit on smaller screens. In
general, given how important they are, they're just too small. This
increases the size and makes the more prominent and easier to hit.
Fix the upload dialogs on the review UI pages.

The upload dialogs didn't contain the Django forms needed to populate
the on-screen forms, due to not using make_review_request_context. This
function (which hopefully we can kill in the future) has been moved into
reviewboard.reviews.context (to prevent a circular dependency) and is
now used in the review UI rendering code.