whatsapp: add autocreation support & force joining

- You can now configure a `autocreate_prefix` in the top-level config.
  If this is set, messages sent to unbridged or new WA groups will
  automatically trigger the creation of a new channel, autogenerated
  based off the group's subject.
- You can also set `ensure_joined` in s2s config to issue a SVSJOIN
  before every message sent to a group's bridged channel, to ensure that
  your admin user is joined to the channel and can receive the message.
  (Currently this just spams SVSJOIN a lot, which doesn't seem to have
  any negative consequences, although this isn't exactly ideal.)