Create Group & Membership group

Depends #527 (closed)

We add 2 models: Group, and the M2M link Membership, that override the existing Django models

• the User pk becomes an uuid
• the Group pk becomes an uuid

classDiagram
    User --|> Membership
    Membership --|> Group


    class Membership {
      level int
      unique(user, group)
    }

    class Group {
      -String name
      -Bool public
    }

The migration should not create any initial Group nor Membership.

Talk with @erwanrouchet for the PK migration

This merge request should only add/update models through the migration, and expose them in the admin:

• list group
• view memberships from a group
• view memberships from a user