We use PubSub only on our account jid, this use case is what PEP
was made for. If PEP is discovered we know that certain PubSub
features are supported, see: https://xmpp.org/extensions/xep-0163.html#defaults
The current check for <feature var='http://jabber.org/protocol/pubsub'/>
is pretty useless, as it just tells us that there is a PubSub implementation
but not much about the features. Only `publish` and `subscribe` are MUST
in XEP-0060 which is not enough for our needs.
If there is ever need to discover a generic PubSub implementation
that is not PEP we should check for all the PubSub features we need
instead of only for <feature var='http://jabber.org/protocol/pubsub'/>
Philipp Hörist
André Apitzsch