Sopel does nothing with this beyond emitting an INFO-level log event,
but plugins might do something more interesting with these.

Enabling the `invite-notify` CAP means that the INVITE verb can contain
other nicks, not only Sopel's. The `admin` plugin has to ignore these.

As a small side project, this also adds test cases for the auto-accept
feature. `test_invite_accept_not_us` fails prior to the associated patch
for `sopel/builtins/admin.py`, because the bot blindly joins `#boudoir`
even though `Anne` was the one invited, not TestBot.