R1:1c5503cb292f

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

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