Migrate old AuxiliaryField storage to modern CustomField storage

Summary: Ref T2222. Ref T3886. Differential has a legacy storage table for auxiliary fields; move the data to modern storage.

Test Plan:
  - Ran migration.
  - Verified fields still worked properly afterward (view, edit, etc).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886, T2222

Differential Revision: https://secure.phabricator.com/D8355