feat: link and unlink an administrator's frontend account on the Administrators screen

- golem15.users.manage_backend_links (no default role), en and pl labels
- record actions on summercms.cabana.users: link_frontend_account asks for
  the frontend user's email (exact, case-insensitive, active users only),
  records linked_by and fires the link events; unlink_frontend_account
  (danger) removes the link
- a taken or unknown account answers 422 on the prompt

admin_users_test.go

@@ -49,7 +49,7 @@ func TestAdminUsersTracer(t *testing.T) {
 	if want := []string{
 		"golem15.users.access_users@developer", "golem15.users.access_groups@developer",
 		"golem15.users.access_settings@developer", "golem15.users.impersonate_user@developer",
-		classes.PermissionManagePrivilegedGroups + "@-",
+		classes.PermissionManagePrivilegedGroups + "@-", classes.PermissionManageBackendLinks + "@-",
 	}; !reflect.DeepEqual(codes, want) || classes.PermissionManagePrivilegedGroups != permManagePrivileged {
 		t.Fatalf("permissions = %v", codes)
 	}