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
# XEP-0012: Last Activity / mod_uptime
[Client] Romeo
jid: romeo@localhost
password: password
-----
Romeo connects
Romeo sends:
<iq id='a' type='get' to='localhost'>
<query xmlns='jabber:iq:last'/>
</iq>
Romeo receives:
<iq type='result' id='a' from='localhost'>
<query xmlns='jabber:iq:last' seconds='{scansion:any}'/>
</iq>
Romeo disconnects