No bulk register/unregister APIs

[mpretty-cyro]

Just adding this as a note for something which could be a nice improvement to make - Currently to register for push notifications we need to individually call /register for the current user and then individually call /subscribe_closed_group for each closed group the user is a member of to help reduce the chance that some registrations succeed and others fail (also reduce the overhead of making multiple network requests)

It would be nice if there was a single endpoint we could call which took:

{
  "token": {users push token},
  "pubkey": {users public key},
  "closedGroupPubkeys": [
    {array of the public keys for each closed group the user is a member of}
  ]
}

Additionally it would be nice to add the equivalent for /unregister (as we have the same issue there)