feat: Organisations admin screen with the avatar and the members manager

- controller golem15.user.organisations behind golem15.users.access_users
- list and form (name, slug preset from the name, description, avatar)
- members relation manager: link sets and unlink clears users.organisation_id
- deleting an organisation releases its members first
- Organisation gains Fillable, Rules, MorphName, AttachRelations, BeforeValidate
  and the Members relation; models.Slugify
- navigation side item organisations
- TestAdminOrganisations, TestAdminOrganisationMembers, TestSlugify

admin_groups_test.go

@@ -90,7 +90,7 @@ func TestAdminGroups(t *testing.T) {
 		}
 	}
 	env.expect(t, http.StatusForbidden, http.MethodPost, adminAPI(adminGroupsPath), usersOnly, map[string]any{"name": "Nope " + stamp, "code": "nope-" + stamp})
-	if got := env.sideMenu(t, usersOnly); !reflect.DeepEqual(got, []string{"users=golem15.user.users"}) {
+	if got := env.sideMenu(t, usersOnly); !reflect.DeepEqual(got, []string{"users=golem15.user.users", "organisations=golem15.user.organisations"}) {
 		t.Fatalf("side menu with the users permission = %v", got)
 	}
 	if got := env.sideMenu(t, token); !reflect.DeepEqual(got, []string{"usergroups=golem15.user.usergroups"}) {