R1:dec14136844d

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…