Restrict GET /item-memberships to admins #1961

Getting memberships is used to get available members in a tree, check and view your permission. Let's specialise each of our use and keep membership handling to admins only.

GET memberships: public -> admin

Necessary changes:

• Get available members in chat
• Get contributors (library) - in /informations or in meilisearch index?
• (optional: currently reader cannot leave) An endpoint to leave an item (currently you can view your own item) ref Add a leave space endpoint #330