R1:c01aa794c1f2

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…