Drop the collective-idea for something else, like users+groups

This is a follow up from two issues #367 and #21.

The collective is thought to provide the following:
- clear distinction between set of documents
- allow multiple users to own files; the use-case is a family where everyone should have access to all (most) of the documents
- allow to own the address book

But this concept is not very flexible and seems confusing (see #367 , #21). Maybe (probably) there is a better option hat also provides the above features. This issue exists to explore/discuss these.

A possibility is to migrate to user and groups, as suggested by @MarcSN311 in #21). A collective becomes a "group" and a user can be member of multiple groups. On registration a group with the username is generated to have a start. Most things could stay the same (probably). A user would login only with the user name. The group/context can be switched without re-login. Documents would be owned by a group as before.