Allow custom fields to have validation logic

Summary:
Ref T418. This is fairly messy, but basically:

  - Add a validation phase to TransactionEditor.
  - Add a validation phase to CustomField.
  - Bring it to StandardField.
  - Add validation logic for the int field.
  - Provide support in related classes.

Test Plan: See screenshot.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D7028