R1:2676e91dd8e1

Move sender validation into MailReceiver classes

Summary:
Ref T1205. Finally able to delete a big chunk of this nastiness.

Make MailReceivers responsible for validating senders. For object creation receivers (bugs, conpherences) this just means that users must not be disabled. For other receivers the senders must be able to see the objects, have the right hashes, etc., according to policy.

Test Plan: Added a bunch of test cases (everything except policy). Verified behavior via the Receive test console.