Add "single user mode" which restricts access to users in the admin group

This can be enabled by setting 'singleuser' to true in config.php
Add occ command to enable and disable single user mode
Add template for single user mode message
add documentation for single user config option to config.sample.php
merge master into single-user-mode
Merge pull request #6036 from owncloud/single-user-mode

Add "single user mode"