login: Don't remove the Homeserver schema unconditionally.
The current Homeserver URL parsing logic unconditionally removes the URL schema if it's either "http://" or "https://", then prepends the "https://" schema back immediately after.
While this correctly adds a "https://" schema for an URL without a schema, it disallows connections to a non-TLS host. Synapse, rightfully so, disallows non-TLS connections. Connecting to such a host might be desirable anyway to connect to a logging/debugging reverse proxy such as mitmproxy or to connect to Pantalaimon, a reverse proxy that enables client-transparent E2E encryption.