The context that's assembled and used to render the diff viewer page

(both the HTML-based UI and the diff context API endpoint) has grown

organically since the very earliest days of Review Board, and is a

pretty big hairy mess. In order to bring at least a little bit of sanity

to it, this change adds typing structures so that type checkers can

verify that we're always assigning things correctly.
The only actual change to the content of the context is that we can no

longer end up with non-bool types for the is_draft_diff and

is_draft_interdiff fields.

Ran unit tests.