Add "Global Auth" option to external storage

This adds the "Global Auth" option to the external storage implementation, it is basically done by reverting c2c6caee from https://github.com/owncloud/core/pull/22468.

This was taken from owncloud/core which is AGPL licensed. Commited with "icewind@owncloud.com" to keep the license header signature in sync.

![2016-06-21_14-01-06](https://cloud.githubusercontent.com/assets/878997/16228662/9c243136-37b8-11e6-9700-c1076ddb3d17.png)