Add server name input using .well-known for custom servers

[jryans]

[jryans]

To clarify, a "server name" input would take something like example.com and then do .well-known discovery (instead of asking for HS / IS URLs).

This is related to #8309, but both login and registration have a "server name" input in the latest designs.

[turt2live]

fwiw this is fairly simple to support even without adding a whole new field: we can reuse the custom server selector and support server names in the input fields. Question would be how we do discovery on the identity server (if we even need to?)