Make Maniphest custom fields extend PhabricatorCustomField

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