direct access to restricted topic

[waclaw66]

When an restricted topic is accessed directly via URL like https://ntfy.sh/restricted_topic there is no prompt to enter credentials and topic is shown in the list of subscriptions with endless circular progress and the user doesn't recognize where is the problem. On the other hand, when that restricted topic is subscribed using "Subscribe to topic" button, the window to enter credentials appears. I think the behaviour should be the same in both cases.

[binwiederhier]

This is strongly related to #294, but slightly different, so I'll keep the tickets separate for now.