Fixes: oc-1369

respect coding style

The data supplied is never used in OwnCloud. Removed the call from all the apps, and made the public API function empty.

Create OC_JSON class, for single point of creating json responses.
No real logic change, this just cleans up the code a bit.

Created the following function:
 - checkLoggedIn
 - checkAdminUser
 - redirectToDefaultPage

Removed global vars are DOCUMENTROOT, SERVERROOT, SUBURI, WEBROOT and CONFIG_DATADIRECTORY

for now this only works from inside the media app