v0.13.6

This is a bugfix release. There are also multiple cleanups and minor
improvements to the web UI by @norgeous.

Resolved issues:

* #3262: v0.13.5 -no-upgrade build now requires gcc (by @calmh, reviewed by @AudriusButkevicius)
* #3267: Favicon should not be animated (by @calmh, reviewed by @AudriusButkevicius)
* #3136: Toggling HTTP/HTTPS breaks GUI/API (by @calmh, reviewed by @canton7)
* #3287: Understand older/newer Hello messages (by @calmh, reviewed by @AudriusButkevicius)
* Warnings on relay connectivity issues are reduced (by @scienmind)
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