Release 6.20.0: Properties for subscription sources

[changerequest]

For every subscription source (say com.openexchange.subscribe.crawlers.someService) a corresponding property will have to be set, indicating whether a certain subscription source is to be available or not. These properties are already in effect for all crawlers, but a property for the OXMF based subscription sources has to be added.

microformatSubscription.properties:
com.openexchange.subscribe.microformats.contacts.h ttp=true
com.openexchange.subscribe.microformats.infostore. http=true