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 to deal with this.

Implement `PhabricatorApplicationTransactionInterface`, implicitly moving all the attach actions (task/task, task/revision, task/commit, task/mock) to proper edge transactions.

The cost of this is that the inverse edges don't write transactions -- if you attach an object to another object, only the object you were acting on posts a transaction record. This is sort of buggy anyway already. I'll fix this in the next diff.

Test Plan: Attached tasks, revisions and mocks to a task, then detached them.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9838