Remove blameRevision and revertPlan from the DifferentialRevision schema

Summary:
These fields use auxiliary storage now. Migrate the data and get rid of the
columns in the main table.

• This might take a little while to run, although there are <500k rows so

probably not too long.

• Maybe grab a backup of the table first, if I screwed something up this will

delete the data in these fields.

Test Plan:

• Ran migration locally.
• Browsed Differential.
• Grepped for "revertPlan" and "blameRevision".

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 832