cleanup: Prefer template strings

Template strings are much nicer than string concatenation, so use
them where possible; this excludes translatable strings and any
strings containing '/' (until we can depend on gettext >= 0.20[0]).

[0] https://savannah.gnu.org/bugs/?50920

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/612

js/ui/shellMountOperation.js

@@ -579,7 +579,7 @@ var GnomeShellMountOpHandler = class {
     _setCurrentRequest(invocation, id, type) {
         let oldId = this._currentId;
         let oldType = this._currentType;
-        let requestId = id + '@' + invocation.get_sender();
+        let requestId = `${id}@${invocation.get_sender()}`;
 
         this._clearCurrentRequest(Gio.MountOperationResult.UNHANDLED, {});