Channel deletions cannot be invalidated

[adrastaea]

Currently, any user can delete a channel, and others do not invalidate their deletion on the basis of a role's permissions. We should set up a custom access controller for the orbitdb which hooks into LFA to determine whether to accept deletion operations based on roles. That way a community could configure that only admins can delete channels (or only the admins + the creator of the channel, etc), and when users see an orbitDB entry suggesting they delete a channel that the rules say they should keep, we can compare their signing of that DEL entry against sigchain state and reject the entry.

[holmesworcester]

In the meantime, we should also modify the UI to let all users delete channels, if we have not done this already, to remove any false expectation that only admins can delete channels.