Fix template rendering for 'blank' templates

See http://api.owncloud.org/classes/OCP.AppFramework.Http.TemplateResponse.html `setRenderAs`

> $renderAs		string		admin, user or blank. 
> Admin also prints the admin settings header and footer, user renders the normal normal page including footer and header and blank just renders the plain template

This was previously done in a helper class  https://github.com/owncloud/music/blob/stable5/appframework/http/templateresponse.php#L118 -> https://github.com/owncloud/music/blob/stable5/appframework/core/api.php#L385:L389


cc @Raydiation @DeepDiver1975 @PVince81 @karlitschek 
This needs backport to stable7 because it's a broken behaviour of the public API.