xmpp: Add setup for ejabberd and jwchat
This change adds setup for ejabberd and jwchat. It should remove the need to install / setup these packages in freedombox-setup.
I added a before_install handler to the @package.required decorator as suggested in #130 (closed). It's used to set debconf values before ejabberd and jwchat are installed.
I also added an apache conf file for jwchat (copied from freedombox-setup) that is installed with plinth, and then enabled after jwchat is installed.