Custom fields: Render 'required' for tokenizer fields

Summary: When building a tokenizer-based edit control for a custom field (e.g. a datasource type), preserve a field validation error whilst building edit controls.

Test Plan:

• Create custom datasource field, set it to required
• Observe that 'Required' does not appear next to control
• Apply patch
• Observe 'Required' appears next to control

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17592