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

Review Request #9004 — Created June 8, 2017 and discarded — Latest diff uploaded

Information

Review Board
release-3.0.x

Reviewers

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
reviews.

Some of the larger, more complex views will be converted in separate
changes.

Unit tests pass.

Thoroughly tested every one of the views.