Limit user visibility to collaborators

### Describe the problem

Hello Weblate community,

we run a shared Weblate instance for multiple independent organizations / customers.
Permissions and repositories are separated, but the global user directory (the `/user` endpoint) shows all users to everyone. It is also problematic that *all* users could be suggested when typing in a comment and starting to type a `@username`.

We would like users to see only people they actually collaborate with (same project/repository or direct interaction), while others remain hidden.
As we operate in the EU (Germany), GDPR requirements force us to apply strict visibility minimization.

### Solution brainstorm

#### Solution 1

Only show users in the users list, who have at least one shared project / repository.

Admins should be able to view all users. This could be implemented with a new global configuration option, or a new team role for viewing all users

#### Solution 2

Add an option to completely disable the `/users` endpoint and the user suggestions when typing a comment.

### Describe alternatives you have considered

Hosting one Weblate instance per customer would theoretically be possible, however, this would be a huge maintenance burden.

### Screenshots

_No response_

### Additional context

Thanks for providing such an amazing translation tool!