Summary:
Ref T418. These implementations share no method names, so we can safely just move Maniphest fields into the `PhabricatorCustomField` hierarchy.
Replaces two Maniphest-specific custom field exceptions which nothing catches.
Test Plan: Viewed Maniphest, edited/altered custom fields.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T418
Differential Revision: https://secure.phabricator.com/D6927
