Membership & roles management

Groups have members, and members can have roles, such as owner or administrator, giving them different permissions on the group. Members can join freely, with moderation or on invitation. None of this is specified in Solid yet, but we are going to work closely with elf Pavlik to integrate this into SAI.

Thoughts on this subject: https://pad.lescommuns.org/sD5H7KynSoud-ygSd_4G2Q

## To do
- [ ] Manage membership (Allow users to join the group, freely or with moderation)
- [ ] Manage roles
- [ ] Manage permissions associated with roles (install, delete, export...)

## Other things to consider
- Manage teams and allow to share access to part of data
- Work with SAI team to define how the authorization agent will handle all this