general: Rename "application" to "app"

Several places in Shell use the word "application". However,"app"
is a more common term nowadays, so update strings accordingly.

https://gitlab.gnome.org/Teams/Design/initiatives/-/issues/123
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6202

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2614>

js/ui/main.js

@@ -284,7 +284,7 @@ function _initializeUI() {
         messageTray.add(source);
         const notification = new MessageTray.Notification(source,
             _('System was put in unsafe mode'),
-            _('Applications now have unrestricted access'));
+            _('Apps now have unrestricted access'));
         notification.addAction(_('Undo'),
             () => (global.context.unsafe_mode = false));
         notification.setTransient(true);