Version 2: List of metadata documents will not work

[VolkerHartmann]

Describe the bug
Listing all metadata documents always return an empty list.
Even with a valid token.

To Reproduce
Steps to reproduce the behavior:

1. Call '.../api/v2/metadata/'
2. See empty list

Expected behavior
Even without a valid authentication all public metadata documents should be listed.
With a valid authentication all schemas with access should be vivisible.

Additional context
Linked to #558

[github-actions]

Branch issue-560-Version_2_List_of_metadata_documents_will_not_work created!