Convert more of the review request views to class-based.
Review Request #9004 - Created June 8, 2017 and discarded
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, and screenshot/file attachment
Some of the larger, more complex views will be converted in separate
Unit tests pass.
Thoroughly tested every one of the views.