Display default server name in registration

If a default server name is set and the current HS URL is the default HS URL,
we'll display that name in the "your account" text on the registration 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
registration.

src/components/structures/MatrixChat.js

@@ -1887,6 +1887,7 @@ export default React.createClass({
                     sessionId={this.state.register_session_id}
                     idSid={this.state.register_id_sid}
                     email={this.props.startingFragmentQueryParams.email}
+                    defaultServerName={this.getDefaultServerName()}
                     defaultServerDiscoveryError={this.state.defaultServerDiscoveryError}
                     defaultHsUrl={this.getDefaultHsUrl()}
                     defaultIsUrl={this.getDefaultIsUrl()}

src/components/structures/auth/Registration.js

@@ -48,15 +48,20 @@ module.exports = React.createClass({
         sessionId: PropTypes.string,
         makeRegistrationUrl: PropTypes.func.isRequired,
         idSid: PropTypes.string,
+        // The default server name to use when the user hasn't specified
+        // one. If set, `defaultHsUrl` and `defaultHsUrl` were derived for this
+        // via `.well-known` discovery. The server name is used instead of the
+        // HS URL when talking about "your account".
+        defaultServerName: PropTypes.string,
+        // An error passed along from higher up explaining that something
+        // went wrong when finding the defaultHsUrl.
+        defaultServerDiscoveryError: PropTypes.string,
         customHsUrl: PropTypes.string,
         customIsUrl: PropTypes.string,
         defaultHsUrl: PropTypes.string,
         defaultIsUrl: PropTypes.string,
         brand: PropTypes.string,
         email: PropTypes.string,
-        // An error passed along from higher up explaining that something
-        // went wrong when finding the defaultHsUrl.
-        defaultServerDiscoveryError: PropTypes.string,
         // registration shouldn't know or care how login is done.
         onLoginClick: PropTypes.func.isRequired,
         onServerConfigChange: PropTypes.func.isRequired,
@@ -470,6 +475,14 @@ module.exports = React.createClass({
             ) {
                 onEditServerDetailsClick = this.onEditServerDetailsClick;
             }
+
+            // If the current HS URL is the default HS URL, then we can label it
+            // with the default HS name (if it exists).
+            let hsName;
+            if (this.state.hsUrl === this.props.defaultHsUrl) {
+                hsName = this.props.defaultServerName;
+            }
+
             return <RegistrationForm
                 defaultUsername={this.state.formVals.username}
                 defaultEmail={this.state.formVals.email}
@@ -481,6 +494,7 @@ module.exports = React.createClass({
                 onRegisterClick={this.onFormSubmit}
                 onEditServerDetailsClick={onEditServerDetailsClick}
                 flows={this.state.flows}
+                hsName={hsName}
                 hsUrl={this.state.hsUrl}
             />;
         }

src/components/views/auth/RegistrationForm.js

@@ -50,6 +50,10 @@ module.exports = React.createClass({
         onRegisterClick: PropTypes.func.isRequired, // onRegisterClick(Object) => ?Promise
         onEditServerDetailsClick: PropTypes.func,
         flows: PropTypes.arrayOf(PropTypes.object).isRequired,
+        // This is optional and only set if we used a server name to determine
+        // the HS URL via `.well-known` discovery. The server name is used
+        // instead of the HS URL when talking about "your account".
+        hsName: PropTypes.string,
         hsUrl: PropTypes.string,
     },
 
@@ -296,13 +300,19 @@ module.exports = React.createClass({
 
     render: function() {
         let yourMatrixAccountText = _t('Create your account');
-        try {
-            const parsedHsUrl = new URL(this.props.hsUrl);
+        if (this.props.hsName) {
             yourMatrixAccountText = _t('Create your %(serverName)s account', {
-                serverName: parsedHsUrl.hostname,
+                serverName: this.props.hsName,
             });
-        } catch (e) {
-            // ignore
+        } else {
+            try {
+                const parsedHsUrl = new URL(this.props.hsUrl);
+                yourMatrixAccountText = _t('Create your %(serverName)s account', {
+                    serverName: parsedHsUrl.hostname,
+                });
+            } catch (e) {
+                // ignore
+            }
         }
 
         let editLink = null;