Make it a SHOULD that it joins MUCs on retrieval of autojoin='1'
bookmarks, leaves them on autojoin='0' or no autojoin, and does the same
when it’s the one modifying the bookmarks.
PubSub §7.2.2.1 says “If no error occurs and the <retract/> element
included a 'notify' attribute with a value of "true" or "1", then the
service MUST delete the item and MUST notify all subscribers as shown
below.”
This means the publishing entity, not the server or the subscription
state, is responsible for properly enabling notifications to other
resources, which makes no sense.
See https://xmpp.org/extensions/xep-0060.xml#publisher-delete-success-notify
In order not to replace old bookmarks when pushing a new one, we need to
request a good max_items value. This one has been picked as some random
large enough number, bikeshed welcome!
TODO: add support for unlimited values?
