The review request page and diff viewer pages have been producing quite a large
number of queries, and this count goes up as the review/comment counts go up.

This change precomputes a lot of data for the review request and diff viewer pages
in order to reduce those queries. Some things that were previously computed as
individual queries per-review are now filtered and categorized out of single
queries up-front.

Some models, like BaseComment and Review, now have accessors that wrap access to
related objects. By default, these are equivalent to accessing the object directly,
but can be short-circuited by setting special variables on the model, which the
review request and diff viewer pages do.

There were also many places in templates where we only needed the ID of a related
object, but we'd still access the entire object just to get it. Now we use the IDs
directly.

With this change, a small review request has gone from 60 queries (total render time of
506ms) to 24 (180ms). A large one went from 396 queries (3.1s) to 24 (348ms)

The diff viewer on the large change went from 270 queries (2.8s) to 24 (330ms).

Tested with several review requests with lots of moving parts, and even compared HTML output.

See the timing above for examples.

We'll need to test this on reviews.reviewboard.org a fair bit before we release, just to make
sure there isn't any fallout.