Display default server name in forgot password

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 forgot password 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 element-hq/element-web#8763 for
forgot password.

src/components/structures/auth/ForgotPassword.js

@@ -41,20 +41,22 @@ module.exports = React.createClass({
     displayName: 'ForgotPassword',
 
     propTypes: {
+        // 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,
+
         defaultHsUrl: PropTypes.string,
         defaultIsUrl: PropTypes.string,
         customHsUrl: PropTypes.string,
         customIsUrl: PropTypes.string,
+
         onLoginClick: PropTypes.func,
         onComplete: PropTypes.func.isRequired,
-
-        // The default server name to use when the user hasn't specified
-        // one. This is used when displaying the defaultHsUrl in the UI.
-        defaultServerName: PropTypes.string,
-
-        // An error passed along from higher up explaining that something
-        // went wrong when finding the defaultHsUrl.
-        defaultServerDiscoveryError: PropTypes.string,
     },
 
     getInitialState: function() {
@@ -235,18 +237,24 @@ module.exports = React.createClass({
         }
 
         let yourMatrixAccountText = _t('Your account');
-        try {
-            const parsedHsUrl = new URL(this.state.enteredHsUrl);
+        if (this.state.enteredHsUrl === this.props.defaultHsUrl) {
             yourMatrixAccountText = _t('Your account on %(serverName)s', {
-                serverName: parsedHsUrl.hostname,
+                serverName: this.props.defaultServerName,
             });
-        } catch (e) {
-            errorText = <div className="mx_Login_error">{_t(
-                "The homeserver URL %(hsUrl)s doesn't seem to be valid URL. Please " +
-                "enter a valid URL including the protocol prefix.",
-            {
-                hsUrl: this.state.enteredHsUrl,
-            })}</div>;
+        } else {
+            try {
+                const parsedHsUrl = new URL(this.state.enteredHsUrl);
+                yourMatrixAccountText = _t('Your account on %(serverName)s', {
+                    serverName: parsedHsUrl.hostname,
+                });
+            } catch (e) {
+                errorText = <div className="mx_Login_error">{_t(
+                    "The homeserver URL %(hsUrl)s doesn't seem to be valid URL. Please " +
+                    "enter a valid URL including the protocol prefix.",
+                {
+                    hsUrl: this.state.enteredHsUrl,
+                })}</div>;
+            }
         }
 
         // If custom URLs are allowed, wire up the server details edit link.