This is the email refactoring part based on sending and listening to signals. All (core) code related to listening to signals and sending notifications (for now, only by email) will be kept under /notifications. This will pave the way for the introduction of webhooks. 

Diff 1 is described here; subsequent diffs will be described in the change description.

## Diff 1:
Rename review_request_draft_save() to review_request_draft_publish()

Saving is actually already done with review_request_draft_set(). What
this method really does is publishing, so the new name suits it
better.

Question though: what is our deprecation policy? How long do we have to keep the old stuff around? Is there any particular marker in the code using for depracating stuff and notifying users when they use old stuff?

All tests run.