User-to-user data sharing with SAI

SAI allows to grant permissions to applications, but also to other users. This is a powerful features, which would allow us to give more visibility to permissions that are granted. It is also a necessary step to enable groups data sharing (see below).

Thoughts on the subject: https://pad.lescommuns.org/QqSRk7L1ThCo4BMonpVafA

## To do

- [x] Implement AgentRegistry (also link ApplicationRegistrations from here)
- [x] Generate Social Agent Registrations and reciprocal registrations
- [x] Migrate contacts to automatically generate Social Agent Registrations