Fixes element-hq/element-web#7724

The `default_server_name` from the config gets displayed in the "Login with my [server] matrix ID" dropdown when the default server is being used. At this point, we also discourage the use of the `default_hs_url` and `default_is_url` options because we do an implicit .well-known lookup to configure the client based on the `default_server_name`. If the URLs are still present in the config, we'll honour them and won't do a .well-known lookup when the URLs are mixed with the new server_name option. Users will be warned if the `default_server_name` does not match the `default_hs_url` if both are supplied. Users are additionally prevented from logging in, registering, and resetting their password if the implicit .well-known check fails - this is to prevent people from doing actions against the wrong homeserver.

This relies on matrix-org/matrix-js-sdk#799 as we now do auto discovery in two places. Instead of bringing the .well-known out to its own utility class in the react-sdk, we might as well drag it out to the js-sdk.

They are now independent of each other. If both are specified in the config, the user will see an error and be prevented from logging in. The expected behaviour is that when a default server name is given, we do a .well-known lookup to find the default homeserver (and block the UI while we do this to prevent it from using matrix.org while we go out and find more information). If the config specifies just a default homeserver URL however, we don't do anything special.

To give the user a little feedback about something happening. This definitely needs to be improved in the future though.