Support following the .well-known entries for the username's domain

[ulyssa]

This has come up several times in #iamb-users. Some homeservers are placed under a different domain or subdomain than what's in the username, and have .well-known entries set up. Since iamb requires passing in a homeserver URL, some users have mistakenly put in the wrong value, which usually results in HTTP errors from POSTing to the login path.

I'd like to make the homeserver URL config optional, and check for the .well-known entry when it's not provided, fall back to building the URL from the domain, and then erroring out with a message asking the user to provide a value.