Update generated URLs to point to v3

[clokep]

Related to #11079, we generate some URLs internally that point to r0 URLs, some places we do this is for:

• Callback URLs used in SSO
• Attempting to fetch media from remote servers

Maybe other places. We should eventually update these to use v3 URLs. The ones that generate URLs that go back to ourselves should be easy enough (if people have their reverse proxies configured properly), but the ones to other servers are a bit more complicated. For federation we try v2 URLs then fallback to v1 URLs, but that's rather annoying. Not sure if there's something better to do.