R1:fbfcc3753161

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…