Allow users to navigate folders when copying

We already have a pattern for navigation folders and searching for
templates – let’s use it for the copy page too. And I reckon we can
represent services as folders if the user has multiple services they
could copy a template from.

app/models/user.py

@@ -1,6 +1,6 @@
 from itertools import chain
 
-from flask import request, session
+from flask import abort, request, session
 from flask_login import AnonymousUserMixin, UserMixin
 
 from app.utils import is_gov_user
@@ -151,6 +151,10 @@ class User(UserMixin):
     def has_permission_for_service(self, service_id, permission):
         return permission in self._permissions.get(service_id, [])
 
+    def belongs_to_service_or_403(self, service_id):
+        if str(service_id) not in self.services:
+            abort(403)
+
     def is_locked(self):
         return self.failed_login_count >= self.max_failed_login_count