R1:b8a03bc145b1

Workboard Trigger Rule: allow to Add/Remove Subscribers

Summary:
This is a natural expansion of the Workboard Trigger system
to support actions about Subscribers.

Context: even before this change - Workboard users
were already able to move Tasks to Columns with
nice automatisms. For instance, you can already:

- Play a nice Sound
- Change Task Priority
- Change Task Status
- Add or Remove a Task Owner
- Add or Remove a Task Project Tag

With this change, you can also:

- Add or Remove Task Subscribers…