Backup & Restore containers configuration

https://github.com/mozilla/multi-account-containers/pull/1533/files

src/options.html

@@ -62,6 +62,23 @@
         </label>
       </div>
       
+      <h3 data-i18n-message-id="backup"></h3>
+      <div class="settings-group">
+        <fieldset>
+          <legend data-i18n-message-id="restoreLegend"></legend>
+          <input id="containersRestoreInput" type="file">
+          <p><em id="containers-restore-result"></em></p>
+          <p data-i18n-message-id="warningConfigOverride"></p>
+        </fieldset>
+        <fieldset>
+          <legend data-i18n-message-id="saveLegend"></legend>
+          <a id="containers-save-link" href="#" style="display: none;"></a>
+          <button data-i18n-message-id="saveButton" data-btn-id="containers-save-button"></button>
+          <p><em id="containers-save-result"></em></p>
+          <p data-i18n-message-id="noteWontBackupCookies"></p>
+        </fieldset>
+      </div>
+      
       <!--
         TODO
         - Add data-i18n