Change soft logout rehydrate text if there's pending key backups

Fixes element-hq/element-web#10263
Requires matrix-org/matrix-js-sdk#982

src/components/structures/auth/SoftLogout.js

@@ -64,6 +64,7 @@ export default class SoftLogout extends React.Component {
             userId,
             displayName,
             loginView: LOGIN_VIEW.LOADING,
+            keyBackupNeeded: true, // assume we do while we figure it out (see componentWillMount)
 
             busy: false,
             password: "",
@@ -73,6 +74,12 @@ export default class SoftLogout extends React.Component {
         this._initLogin();
     }
 
+    componentWillMount(): void {
+        MatrixClientPeg.get().flagAllGroupSessionsForBackup().then(remaining => {
+            this.setState({keyBackupNeeded: remaining > 0});
+        });
+    }
+
     onClearAll = () => {
         const ConfirmWipeDeviceDialog = sdk.getComponent('dialogs.ConfirmWipeDeviceDialog');
         Modal.createTrackedDialog('Clear Data', 'Soft Logout', ConfirmWipeDeviceDialog, {
@@ -158,9 +165,16 @@ export default class SoftLogout extends React.Component {
                 error = <span className='mx_Login_error'>{this.state.errorText}</span>;
             }
 
+            let introText = _t("Enter your password to sign in and regain access to your account.");
+            if (this.state.keyBackupNeeded) {
+                introText = _t(
+                    "Regain access your account and recover encryption keys stored on this device. " +
+                    "Without them, you won’t be able to read all of your secure messages on any device.");
+            }
+
             return (
                 <form onSubmit={this.onPasswordLogin}>
-                    <p>{_t("Enter your password to sign in and regain access to your account.")}</p>
+                    <p>{introText}</p>
                     {error}
                     <Field
                         id="softlogout_password"

src/i18n/strings/en_EN.json

@@ -1586,8 +1586,10 @@
     "You can now close this window or <a>log in</a> to your new account.": "You can now close this window or <a>log in</a> to your new account.",
     "Registration Successful": "Registration Successful",
     "Create your account": "Create your account",
+    "Failed to re-authenticate due to a homeserver problem": "Failed to re-authenticate due to a homeserver problem",
     "Failed to re-authenticate": "Failed to re-authenticate",
     "Enter your password to sign in and regain access to your account.": "Enter your password to sign in and regain access to your account.",
+    "Regain access your account and recover encryption keys stored on this device. Without them, you won’t be able to read all of your secure messages on any device.": "Regain access your account and recover encryption keys stored on this device. Without them, you won’t be able to read all of your secure messages on any device.",
     "Forgotten your password?": "Forgotten your password?",
     "Cannot re-authenticate with your account. Please contact your homeserver admin for more information.": "Cannot re-authenticate with your account. Please contact your homeserver admin for more information.",
     "You're signed out": "You're signed out",