implement client support for muc push

Staying connected to a MUC room hosted on a remote server can be challenging.

If a server reboots it will usually send a shut down notification to all
participants. However even if a client knows that a server was shut down it
doesn’t know when it comes up again. In some corner cases that shut down
notification might not even be delivered successfully leaving the client in a
state where it thinks it is connected but it really isn’t.

The possible work around implemented in this commit is to register the clients
full JID (user@domain.tld/Conversations.r4nd) as an App Server according to
XEP-0357 with the room. (Conversations checks for the push:0 namespace on the
room.)

After cycling through a reboot the first message send to a room will trigger
pubsub notifications to each registered full JID. This event will be used to
trigger a XEP-0410 ping and if necessary a subsequent rejoin of the MUC.

If the resource has become unavailable during down time of the MUC server the
user’s server will respond with an IQ error which in turn leads to the MUC
server disabling that push target.

Leaving a MUC will send a `disable` command. If sending that disable command
failed for some reason (network outage) and the client receives a pubsub
notification for a room it is no longer joined in it will respond with an
item-not-found IQ error which also disables subsequent pushes from the server.

Note: We 0410-ping before a join to avoid unnecessary full joins which can be
quite costly. Further client side optimazations will also surpress pings when
a ping is already in flight to further save traffic.