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

If you need inspiration, this feature is useful for adding
more eyes on a given work area; lighten the notifications
of certain people after certain workflows have already been
done, to increase happiness and mitigate Burnout.

If your goal is to oppress yourself or your coworkers - of
course you can also use this feature to increase the Burnout
of yourself or your coworkers, adding random people as
Subscriber.

Note: every trigger action can be confusing in general, but
it is the fault of Triggers in general and not related to
this specific feature - that is totally loveable,
just like you.

Closes T15162

Test Plan:

• Workboard > Column > Create Trigger
• Add Action > Add Subscriber > (your best friend who hates NodeJS)
• Add Action > Remove Subscriber > (your antagonist who hates PHP)
• Move some Tasks there and here, and note that Add/Remove works

Reviewers: O1 Blessed Committers, Cigaryno, aklapper

Reviewed By: O1 Blessed Committers, Cigaryno, aklapper

Subscribers: aklapper, speck, tobiaswiese, Matthew, Cigaryno

Tags: #workboard

Maniphest Tasks: T15162

Differential Revision: https://we.phorge.it/D25080