Group creation and invitation

We should allow users to create groups and invite other people in those groups.

• Add a permission for group creation, and allow people with this permission to create new groups (but not manage all the groups)
• Use the ldap owner attribute to list the group administrators.
• Group administrators can edit the group description.
• Group administrators can invite other people in the group.
• Invitation is made by sending an invitation link by email.
• People can leave their groups.