Migrate Audit inline comments to new storage

Summary:
Ref T4896. This is substantially identical to the process which Differential followed, and mostly copied from the original Differential migration and the Differential proxy object.

Basically, we move all the data over but the application can't tell, and the same APIs do reads and writes to the new table.

Test Plan:

• Browsed UI before migrating, everything looked fine (but no inlines).
• Ran migration.
• Verified draft and published comments survived migration.
• Added a draft.
• Previewed draft.
• Submitted draft.
• Viewed standalone with drafts and published comments.
• Sanity checked data in database, didn't see anything unusual.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10018