Instances with only onion domains
Add the ability for instances to have only an onion domain without needing a DNS domain as well.
This is important because:
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onion domains can't be taken away as per https://pl.smuglo.li/notice/9uXaOtkD0ZBCfkAxPM
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onion domains can't be re-registered by someone else and pretend to be the original instance
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onion domains protect against doxing, especially useful when considering the nature of some fedi members
This should be possible without making any changes to the standard. Just do any request via tor (and without tls) when seeing an .onion domain.