Short circuit if already a team member

It would be confusing if people got invited twice, so let’s tell people
if someone’s already been invited.

app/main/views/manage_users.py

@@ -65,6 +65,11 @@ def invite_user(service_id, user_id=None):
 
     if user_id:
         user_to_invite = User.from_id(user_id)
+        if user_to_invite.belongs_to_service(current_service.id):
+            return render_template(
+                'views/user-already-invited.html',
+                user_to_invite=user_to_invite,
+            )
         form.email_address.data = user_to_invite.email_address
     else:
         user_to_invite = None