MUC: Don't default component admins to being room owners
This change has various technical and social benefits. If ownership of a MUC
is really needed, it can be gained using the 'Set affiliation' ad-hoc command
or prosodyctl shell.
Example client incompatibility with the old behaviour:
- https://github.com/monal-im/Monal/issues/1085
# Tombstones
[Client] Romeo
jid: romeo@localhost
password: password
[Client] Juliet
jid: juliet-tombstones@localhost
password: password
---------
Romeo connects
Juliet connects
Juliet sends:
<iq type="set" id="bye">
<query xmlns="jabber:iq:register">
<remove/>
</query>
</iq>
# Scansion gets disconnected right after this with a stream error makes
# scansion itself abort, so we preemptively disconnect to avoid that
# Juliet receives:
# <iq type="result" id="bye"/>
Juliet disconnects
Romeo sends:
<presence type="probe" to="${Juliet's JID}"/>
Romeo receives:
<presence type="error" from="${Juliet's JID}"/>
Romeo receives:
<presence type="unsubscribed" from="${Juliet's JID}"/>