do not show 'Add app' and 'More apps' for themed ownCloud

Makes the »Add your app« and »More apps« elements in the App management not show up in themed versions.
Check it @karlitschek @DeepDiver1975 @schiesbn  

Maybe it makes even more sense for the future to have this as a config switch much like a custom 3rd party appstore. On/off and custom links for these functions.

(Also needs to be backported to stable6 for EE.)