[6.0.5] Fix merging of external storage configurations

This is a backport of https://github.com/owncloud/core/pull/7888

Merging of configurations is whenever the same config is available for
multiple users/groups, in which case the config is considered as a
single one by the UI, and shows multiple users/groups selected.

Fixed merging logic to make sure that class, mount point and options are
the same before merging them.

Fixed merging to work correctly when the same mount point path is used
for separate users and configs. These are now correctly shows in the UI
as separate entries.