Load authentication apps to get users from all backends - fixes #7019
Added isLocal() method to storage, used for xsendfile

Added isLocal() method to Storage to find out whether the storage is
local or not.
This method is used for the x-sendfile logic to find out whether to add
the headers.
Merge pull request #7075 from owncloud/quota-storagexsendfile

Added isLocal() method to storage, used for xsendfile
Merge pull request #7085 from owncloud/quick-fix-7019-master

Load authentication apps to get users from all backends
Merge branch 'master' into backgroundjob-public
Remove internal methods from the public interface
Additional phpdoc
Add unit tests for JobList