js: Don't use templates in files with translations

xgettext gained some support for template strings, and no longer
fails when encountering '/' somewhere between backticks.

Unfortunately its support is still buggy as hell, and it is now
silently dropping translatable strings, yay. I hate making the
code worse, but until xgettext really gets its shit together,
the only viable way forward seems to be to not use template
strings in any files listed in POTFILES.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1014

js/ui/overviewControls.js

@@ -127,7 +127,7 @@ class SlidingControl extends St.Widget {
     }
 
     _getSlide() {
-        throw new GObject.NotImplementedError(`_getSlide in ${this.constructor.name}`);
+        throw new GObject.NotImplementedError('_getSlide in %s'.format(this.constructor.name));
     }
 
     _updateSlide() {