Feature: Following a 'club' profil makes following its friend list

**Community discussion about this feature (mandatory)**
[Link to where this feature has been validated](https://forum.camptocamp.org/t/groupes-dutilisateurs/290469)
The idea is to start creating sub-communities in a light way.

**Describe the solution you'd like**
When following a profil of the type 'club', you follows his activities **and** the activities of its friends (for the feed preference).
A club is hence managed via the club profil (add friends), and non-members can follow the activities.

Option : show the friends of a 'club' in its profil.

Start by reviewing the validated forum discussion and the existing follow, feed-preference, and club-profile behavior. Define how following a club includes its friends' activities, then verify the optional display of the club's friends in its profile.

Written by the indexing model from the issue text.