Previously, anyone (even across federation) could invite you to a
room and the bot would join. It may not have provided a useful purpose,
but it still joined all rooms it was invited to.
We now only join rooms when we're invited by a person who is actually
allowed to use the bot.
Fixes https://gitlab.com/etke.cc/postmoogle/-/issues/17
Checking using `settings.Allowed` is odd. Not all commands are related
to setting configuration settings. Admin commands are coming in the
future, for which this is certainly not the case.
We now do access checks early on (during command processing), so command
handlers can be clean of access checks. If we're inside of a command
handler, the user is privileged to run it.
