Proposal: support `.onion` domains

I would like to propose and potentially contribute a PR to allow Vaultwarden to work cleanly with Tor onion service addresses. There are two intended use cases: 1. Onion-only access, where DOMAIN is a .onion URL. 2. Clearnet plus Tor access, where DOMAIN remains the primary clearnet URL and one or more additional .onion URLs are configured for Tor access. Example onion-only setup: Example clearnet plus onion setup: Proposed scope: • allow valid v3 .onion URLs in DOMAIN; • add support for one or more additional .onion URLs via a separate setting; • allow • do not add support for arbitrary additional clearnet domains; • preserve existing behaviour for ordinary clearnet HTTP URLs; • use the matching incoming request host/proto where appropriate when generating links, redirects, canonical URLs, CORS, allowed origins, WebSocket origins, icons, attachments, and relevant web vault/API responses; • keep email/template…