RFE: Limit group size reported by roster groups

[MarcelWaldvogel]

RFE based on jsxc/xmpp-cloud-auth#73:

Add an admin setting, which selects the largest group size that should be reported by the roster function. Any group larger than this (think 'employees' and similar) would not be reported. This would not only be good for the users, but also for system performance (evaluating and/or sending a large group list on every login is not really desirable).

Setting the value to 0 would disable automatic roster groups in a most natural fashion.