Limit access for user to main person and relatives

[Luensche]

Is your feature request related to a problem? Please describe.
I have a relatively large family tree and would like to give other family members access. However, as living persons are maintained in the family tree, I would only like to grant access to the user's own person and their ancestors/descendants + persons directly related by marriage. Other people can appear anonymised with initials, so that it is clear that there are other people, but without dates etc.

Describe the solution you'd like
When creating a user, it should be possible to select a main person. If this is done, it should not be possible to change this in the user's own settings.
All direct ancestors+descendants plus relatives should be displayed normally, all others only with initials.

Describe alternatives you've considered
Multiple family trees on one instance, which have to be synced manually.

[DavidMStraub]

Related discussion: https://gramps.discourse.group/t/restricting-parts-of-the-tree-from-distant-relatives/6661/7

[DavidMStraub]

Also related: #536