Extend homeserver autodiscovery to 3pids

[turt2live]

Extend this: matrix-org/matrix-react-sdk#2227

That supports entering an mxid into the login field, however it is likely that someone will also want to punch in their email or even phone number to log in. We should be able to accomplish this by doing a lookup to the default IS for Riot and then proceeding with autodiscovery from the found mxid. This has the disadvantage of lookup results failing if the IS doesn't match, however decentralized identity would solve that problem.

With this change, it'd also be likely that we could finally remove the dropdown to pick which identifier to log in with.