[WIP] Ext storage app framework + proper REST

Tried to make the external storage app more meaningful.

But I hit against hard blocks like this: https://github.com/owncloud/core/blob/master/apps/files_external/js/settings.js#L136
Calling "removeMountPoint.php" for EVERY user ?

Also the lack of "id" makes it impossible to continue.
Will try and introduce a proper id in a separate PR first...