Prefer users with similar pools for user recommendations

Instead of recommending just random users, prefer users with a similar set of selected pools. Attempt to include some kind of randomization to avoid the recommendation from being too static and to increase the number of recommended users. This may cause less suitable users to be recommended.

As in #50 (closed) do not show users who are already friends with the requesting user. The API endpoint should be unaffected by this.

Note: Since questions can be assigned to universities but pools can't, it's not enough to check if users have pools in common, but you also have to check that pools have >=2 questions both users can access. You may use https://gitlab.akamu.de/akamu/game-server-go/-/blob/development/endpoint/pool/query.go#L83