Don't show 'unpin' for persistent widgets

This will mostly fix https://github.com/vector-im/element-web/issues/15139
although it could still break in a race condition.

src/components/views/elements/AppTile.js

@@ -841,7 +841,7 @@ export default class AppTile extends React.Component {
             contextMenu = (
                 <ContextMenu {...aboveLeftOf(elementRect, null)} onFinished={this._closeContextMenu}>
                     <WidgetContextMenu
-                        onUnpinClicked={this._onUnpinClicked}
+                        onUnpinClicked={ActiveWidgetStore.getWidgetPersistence(this.props.app.id) ? null : this._onUnpinClicked}
                         onRevokeClicked={this._onRevokeClicked}
                         onEditClicked={showEditButton ? this._onEditClick : undefined}
                         onDeleteClicked={showDeleteButton ? this._onDeleteClick : undefined}