[omemo] Allow mark non-anonymous MUC as public
Some public MUCs are non-anonymous (e.g. devel@conference.pidgin.im, open_chat@conference.igniterealtime.org, movim@conference.movim.eu), thus OMEMO plugin decides that it might be enabled [1] and queries devicelists from actually unknown random people. Let's introduce an option to mark a MUC as public to avoid needless operations and filling database with irrelevant data.