[XEP-0313]: wip - Message archiving

Support for server-side history. Currently no archiving preferences or message
queries supported. Just part of the XEP implemented so far.
[XEP-0313]: wip - Basic support for preferences

Not possible to set/get via <iq> commands yet, but once they are set in
database, settings are respected.
[XEP-0313]: wip - Finished preferences

It is now possible to set & get archiving preferences using <iq> as specified
in XEP-0313 and these settings correspond settings in database.