cleanup: Fix logic for implicit subscriptions

[MaartenGrothus]

Implicit subscriptions should be removed once the respective implicator no longer holds.

Exemplarily, for a club newsletter, this ensures that people which became subscribed due to their membership are unsubscribed once it is terminated.

In doing so, this change removes the implicits_only subscription policy, which is redundant to subscribable with this change.

Another way to go about this would be to add another option for an implicit subscription to be "sticky", i.e. to persist after the implicator is lost if the person still has SubscriptionPolicy.subscribable.
The best way to go about this would be to introduce a new, orthogonal flag, is_sticky, which can be passed to _apply_cleanup and would then replace and policy.is_implicit() with and not is_sticky. In cdedb, this would not be used.

What do you think?

[larsesser]

Another way to go about this would be to add another option for an implicit subscription to be "sticky", i.e. to persist after the implicator is lost if the person still has SubscriptionPolicy.subscribable. The best way to go about this would be to introduce a new, orthogonal flag, is_sticky, which can be passed to _apply_cleanup and would then replace and policy.is_implicit() with and not is_sticky. In cdedb, this would not be used.

I think this may be useful, but since we have no imediate use case, I would vote for postponing this. Maybe open up an issue.