Provide builtin definitions for "Blame Revision" and "Revert Plan" fields

Summary:
This is just to ease transitions for any installs which use these fields (e.g.,
Facebook). I'll write some docs and a migration script once this stuff is a
little more solid, too.

Depends on D800.

Technically these are "better" than the current fields since they show up other
places than the edit screen (derp derp).

Test Plan: Created a field selector which provides these; verified they work by
typing stuff into them and saving the revision.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, jungejason
Differential Revision: 801