Inform the client if it subscribes to a new topic

[karmanyaahm]

• Reject Matrix with 200 and rejected pushkey instead of returning 5xx
• Set UnifiedPush 'can't publish' error to 404
• Implement a NeverSubscribed() method in topics to keep track of new topics
• add the "new_topic" param to the open message
• Return new_topic in the open message when any of the list of topics subscribed to is new. Not the most information, but the likelihood of topics from the same client have different statuses is so tiny and rare, that clients can resubscribe all to be safe.

Deployment I think the Android app needs to be released first, before the server is deployed, so the client can deal with the case of re-registration on 404.

Paired with https://github.com/binwiederhier/ntfy-android/pull/70 Closes https://github.com/binwiederhier/ntfy/issues/664

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.

Project coverage is 65.85%. Comparing base (bb4b5d2) to head (ec79778). Report is 172 commits behind head on main.

Files	Patch %	Lines
server/server.go	55.55%	3 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #979 +/- ## ========================================== + Coverage 65.82% 65.85% +0.02% ========================================== Files 53 53 Lines 7737 7746 +9 ========================================== + Hits 5093 5101 +8 - Misses 1799 1801 +2 + Partials 845 844 -1 ```

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