disallow blocked users from the system

[ruslandoga]

Right now blocked users can access the system (they can log in, connect to socket, etc.). This PRs removes that ability.

Scenario 1, user is blocked while online

1. Accounts.block_user/1 is called that blocks the user and removes all session tokens
2. At the call site of Accounts.block_user/1, session tokens are iterated and for each connected socket disconnect command is sent
3. Blocked user disconnects from backend, iOS tries to reconnect, gets 403 in response, and proceeds to log out like in test4
4. On sign-in-with-apple screen user tries to log in, gets 403 response again, this time meaning that their account is no longer active and iOS shows a banner with support contacts

Scenario 2, user is blocked while offline

1. Accounts.block_user/1 is called that blocks the user and removes all session tokens
2. When connecting to the socket some time later, ios gets 403 response for the provided token, proceeds to log out
3. same as 4. in scenario 1