Fixes https://github.com/vector-im/riot-web/issues/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 https://github.com/matrix-org/matrix-js-sdk/pull/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.
