ReviewRequest.get_draft() fetched the draft every time it was called.

This was somewhat acceptable once, but with the introduction of actions

and some fields, we ended up calling this a lot, leading to an excess

of database queries on the review request pages. This also meant that

all the callers got separate instances, which could in theory cause some

problems with state synchronization and data loss during mutation.
We now cache this on the ReviewRequest object, so that repeated

calls will return the same draft instance. We clear this when calling

ReviewRequest.clear_local_caches(), which includes when the draft is

deleted or published.
The codebase was audited to make sure this wouldn't result in any

obvious problems, though it does mean that callers that may be working

with multiple instances of the same review request (such as API unit

tests) must be careful to clear caches if re-querying for a draft.

All unit tests pass.
Spent some time running with this patch locally, testing the typical

functionality of posting changes, updating changes, publishing, deleting.

Never hit any issues. It will need more production testing before

release.