R1:ace1feb70220

Implement PhabricatorApplicationTransactionInterface on ManiphestTask

Summary:
Ref T5245. A very long time ago I had this terrible idea that we'd let objects react to edges being added and insert transactions in response.

This turned out to be a clearly bad idea very quickly, for like 15 different reasons. A big issue is that it inverts the responsibilities of editors. It's also just clumsy and messy.

We now have `PhabricatorApplicationTransactionInterface` instead, which mostly provides a cleaner way…