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 binwiederhier/ntfy-android#70
Closes #664

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (bb4b5d2) 65.82% compared to head (ec79778) 65.85%.

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

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Additional details and impacted files

@@            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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