Migrate auxiliary field storage to common field storage

Summary: Ref T418. Moves data from the Maniphest-specific table to the general one. This patch is a bit gross, but mostly about getting the reads and writes aimed correctly. Future patches will clean things up.

Test Plan: Migrated data across formats. Verified it survied the migration. Viewed and edited tasks' custom fields.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D6999