Collections: Allow user management

[MrSerth]

Currently, there is no mechanism to remove another member from a collection. While we can invite someone else, they can decide to join and leave, no one else can remove an accepted member (even not the person who sent the invite).

While I am aware that this limitation is due to the fact that collections don't have a dedicated "admin", we should discuss how to improve the situation. Potentially, we can combine collections and groups more, so that the user management is only done once.

[kkoehn]

We talked about groups and collections a while ago and it resulted in #781. The only thing left is actually linking collections with groups.

An idea could be to remove collection sharing completely and only allow the owner to edit the collection. The owner can be a user or a group. To make it even more clear we could enforce, that a collection can only have one owner: either a collection or a group. If I as a person create a collection, I can transfer it to one of my groups and a group admin can transfer it to another group or user. That way it would always be transparent who has which rights.

[MrSerth]

I remember we chatted about #781 and linking collections to groups. I am still in favor of this and mainly created this ticket not to forget about the issue.

Regarding the specific implementation, I am very open to any idea. Your suggestion sounds good, and linking it to an individual or a group at any time probably makes sense (you meant a person, rather than "a collection", right?). Also, I think that this design would be relatively clear to communicate, since user management is only done one.