Display default server name in login

If a default server name is set and the current HS URL is the default HS URL,
we'll display that name in the "sign in to" text on the login form.

This can be a bit more user friendly, especially when the HS is delegated to
somewhere such as Modular, since you'll then see "example.com" instead of
"example.modular.im", which you have no direct relationship with as a user.

This is the key bit of https://github.com/vector-im/riot-web/issues/8763 for
login.

src/components/structures/MatrixChat.js

@@ -1924,6 +1924,7 @@ export default React.createClass({
                 <Login
                     onLoggedIn={Lifecycle.setLoggedIn}
                     onRegisterClick={this.onRegisterClick}
+                    defaultServerName={this.getDefaultServerName()}
                     defaultServerDiscoveryError={this.state.defaultServerDiscoveryError}
                     defaultHsUrl={this.getDefaultHsUrl()}
                     defaultIsUrl={this.getDefaultIsUrl()}