imageListToDbUrl should be used if the URL is going to be saved to the database, to avoid saving nothing in case at the moment of saving the user preference is to not show images.
Now only the subscriptions from the selected group by the user will be
loaded.
Also add an option to decide how much time have to pass since the last
refresh before the subscription is deemed as not up to date. This helps
when a subscription appear in multiple groups, since updating in one
will not require to be fetched again in the others.