Create field "Open to contribution by" for services

[trugwaldsaenger]

This field should allow to show, who can contribute ressources to a repository or other service.

Possible values should be:

• to everyone
• to everyone after registration
• to instituitonal members

Ideally this field should allow to identify repositories (or other content sources), which are open for everybody, since it is still a common problem for authors without institutional affiliation to find a platform, where they can deposit their materials.

[trugwaldsaenger]

@gandalfar I will quality check the wording with the team and tell you, when it is ready for implementation!