Prevent updating a closed review request through the UI.

A closed review request still had a fully-functioning Update menu,
which allowed new diffs and file attachments to be added without any
visible draft banner. While these would internally be part of a draft
and wouldn't be visible to any other users, it was confusing and
conflicted with the fact that all other editable components were
disabled.

The "X" buttons for deleting file attachments and screenshots was also
being shown, but was non-functional. It too has been removed.

Reviewed at https://reviews.reviewboard.org/r/6268/