R1:b21b73b8dd35

Expand Revision transaction API to allow actions to vary more broadly based on the viewer and revision state

Summary:
See PHI1810. Build toward support for "Request Review" by non-authors on drafts, to forcefully pull a revision out of draft.

Currently, some action strings can't vary based on revision state or the current viewer, so this "pull out of draft" action would have to either: say "Request Review"; or be a totally separate action.

Neither seem great, so allow the labels and messages to vary…