Unsubscribe from identity contracts after leaving group

[taoeffect]

Problem

If we are kicked out of a group, or if we leave a group, we remain subscribed to the identity contracts that were in that group.

This is a waste of resources if we are no longer interacting with these contracts.

Solution

For all of the identity contracts with whom we do not have any open DMs, unsubscribe from them.

Note: the e2e-protocol branch is merging the mailbox.js contract into the identity.js contract. So for this PR do not worry about unsubscribing from the corresponding mailbox contracts. Just unsubscribe from the identity contracts with whom we do not have any other open interactions (e.g. DMs) with.

EDIT: it is best to probably wait until the e2e-protocol branch is merged. See PR #1521.

EDIT2: I think this PR would be best addressed by using reference counting (see #1249), but it can also be addressed by other means (see @Silver-IT's #1885)