Convert more of the review request views to class-based.

Review Request #9021 - Created June 16, 2017 and submitted

Christian Hammond
Review Board

This converts a handful of the smaller review request views to
class-based views, documenting them in the process. This covers the root
view, diff downloads, bug-related views, review request infobox, and
screenshot/file attachment reviews.

Tested each of the views as authenticated and anonymous users. Tested
each code path.

Christian Hammond
David Trowbridge
Christian Hammond
Review request changed

Status: Closed (submitted)

Change Summary:

Pushed to release-3.0.x (59cf8fb)