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