Release 6.20.0: Allow administrators to disable messaging or specific messaging services

[changerequest]

The following properties were added. They can be configured with the config cascade:

messaging.properties:

com.openexchange.messaging.enabled=true|false

facebookmessaging.properties:
com.openexchange.messaging.facebook=true|false

rssmessaging.properties:
com.openexchange.messaging.rss=true|false

twittermessaging.properties:
com.openexchange.messaging.twitter=true|false