Add storage priority support

Each storage backend has a default priority, assigned to any system mounts
created in ownCloud. mount.json can be manually modified to change these
priorities.

The priority order is as follows:
 * Personal
 * User
 * Group
 * Global

Within each mount type, the mount with the highest priority is active.

The storage backend defaults were chosen to be the following:
 * Local - 150
 * Remote storage - 100
 * SMB / CIFS with OC login - 90