Respect token limits for "Assign to" and custom datasource fields in Herald

Summary:
See PHI173. Currently, Herald has an "Assign to" action for tasks, and you can specify custom fields with datasource values (like users or projects) that have a limit (like 1 "Owner", or 12 "Jury Members").

Herald doesn't support these limits right now, so you can write [ Assign to ][ X, Y, Z ]. This just means "Assign to X", but make it more clear by actually enforcing the limit in the UI.

Test Plan:

• Created a "projects" custom field with limit 1.
• Tried to create actions that 'assign to' or 'set custom field to' more than one thing, got helpfully rebuffed by the UI.
• Created an "add subscribers" action with more than one value.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18887