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. Testing Done: - Ran unit tests.