New endpoint ListGroupMemberships

A connected user must be able to list all the memberships of a group:

• GET /api/v1/group/<uuid>/members
• list all the memberships with their level, and user email / name

Payload:

"number": 1,
"count": 1,
"next": null,
"previous": null,
"results": [
  {
    "level": 99,
    "email": "some_email",
    "display_name": "user_name",
    "id": "UUID"
  }
]