Provide a more general "Author" transaction for Differential

Summary:
Ref T13628. Currently, Differential has a "Commandeer" action, but no way to edit the author otherwise.

This is largely archaic: there is no reason to prevent editing the author, and this makes it difficult to undo mistakes if you commandeer by accident.

Instead, provide a normal "Author" field and a "Foist Upon" action, similar to the "Owner" and "Claim/Assign" fields in Maniphest.

Test Plan:

• Applied author transactions as the old author ("foisted"), the new author ("commandeerd"), and an arbitrary third party ("changed author").
• Tried to unassign author, etc.
• Viewed stories in feed and transaction timeline.
• Saw sensible automatic reviewer changes.
• Used existing "Commandeer" action, which is unchanged.
• Called "transaction.search" and saw reasonable transaction values.

Maniphest Tasks: T13628

Differential Revision: https://secure.phabricator.com/D21591