Support "Set X to" as an action in Herald for tokenizer/datasource custom fields

Summary:
See PHI173. Adds custom field support for Herald actions, and implements actions for "Datasource/Tokenizer" fields.

The only action available for now is "set field to...". Other actions ("Add values", "Remove values") might make sense in the future for these fields, but there's currently no use case. For most other field types (text, select, checkbox, etc) only "Set to" makes sense.

Test Plan:

• Added a "datasource" custom field to the custom field definition in Config.
• Added a "if field is empty, set field to default value X" rule to Herald.
• Created a task with a nonempty field: no Herald trigger.
• Created a task with an empty field: Herald fired.
• Reviewed rule and transcripts for text strings.

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Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18784