muc: only do disco on succesfully entering a room

Sending a disco#info <iq> on being banned from a room is wasteful, and
when _creating_ a room dino asks too soon, receives an error, and gives
up on syncing the room metadata giving subtle, hair-tearing inconsistencies
because flag.has_room_features() returns false for everything.

It's barely noticeable, because if you're banned from a room you
can't query it anyway (though in that case it's still a wasted <iq>),
and if you're creating the room all its fields are empty by default.

I noticed this because I'm experimenting with https://modules.prosody.im/mod_muc_defaults;
dino incorrectly shows moderation off when it should be on in newly
created rooms.

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