R1:1e93b49b1bf2

Allow custom actions in Differential to explicitly override "accept" stickiness

Summary:
See PHI431. Ref T13102. An install is interested in a custom "non-sticky" accept action, roughly.

On the one hand, this is a pretty hacky patch. However, I suspect it inches us closer to T731, and I'm generally comfortable with exploring the realms of "Accept Next Update", "Unblock Without Accepting", etc., as long as most of it doesn't end up enabled by default in the upstream.

Test Plan:
- Accepted and updated…