Added some new API functions to ReviewRequest and Review. These functions give us a standard way to grab the review request's draft or the user's pending review and to grab a user's pending reply to a review.

This also switches to some API functions in other places instead of hard-coding queries, and removes redundant saves immediately after a get_or_create.

It also fixes a couple potential bugs where we were querying a review to delete that may not be the review we expect. We expect to work on actual reviews, not replies, and the query in question may give us a reply in some cases.

Unit tests pass.