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 straightforward and low-impact.
  - This migration promotes "auxiliary field" data to the new CustomField/StandardField format, so that's not a straight migration either. The formats are very similar, though.

Broadly, this takes the same attack that the auth migration did: proxy all the code through to the new storage. `ManiphestTransaction` is now just an API on top of `ManiphestTransactionPro`, which is the new storage format. The two formats are very similar, so this was mostly a straight copy from one table to the other.

Test Plan:
  - Without performing the migration, made a bunch of edits and comments on tasks and verified the new code works correctly.
  - Droped the test data and performed the migration.
  - Looked at the resulting data for obvious discrepancies.
  - Looked at a bunch of tasks and their transaction history.
  - Used Conduit to pull transaction data.
  - Edited task description and clicked "View Details" on transaction.
  - Used batch editor.
  - Made a bunch more edits.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7068