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_users_test.go

@@ -69,7 +69,7 @@ func TestAdminUsersTracer(t *testing.T) {
 	}
 	adminInto(t, env.expect(t, http.StatusOK, http.MethodGet, adminAPI("/navigation"), token, nil), &nav)
 	if len(nav) != 1 || nav[0].Code != "user" || nav[0].Order != 555 || nav[0].Controller != "golem15.user.users" ||
-		len(nav[0].SideMenu) != 1 || nav[0].SideMenu[0].Code != "users" {
+		len(nav[0].SideMenu) != 2 || nav[0].SideMenu[0].Code != "users" || nav[0].SideMenu[1].Code != "organisations" {
 		t.Fatalf("navigation = %+v", nav)
 	}
 	nav = nil