R1:7abe9dc4c078

Migrate all Maniphest transaction data to new storage

Summary:
Ref T2217. This is the risky, hard part; everything after this should be smooth sailing. This is //mostly// clean, except:

- The old format would opportunistically combine a comment with some other transaction type if it could. We no longer do that, so:
- When migrating, "edit" + "comment" transactions need to be split in two.
- When editing now, we should no longer combine these transaction types.
- These changes are pretty…