Improved notifications cleaning UI with set operations (#109)

* added notification cleaning drawer

* bugfix

* fully implemented set operations for notif cleaning

* i18n for notif cleaning drawer & improved logic slightly. Also added a confirm dialog

* - notif dismiss "overlay" now shoves the notif aside to avoid overlap
- added focus ring to header buttons
- removed notif overlay entirely from DOM if mode is disabled

* removed comment

* CSS tuning - inconsistent division lines fix

app/javascript/styles/application.scss

@@ -1,5 +1,6 @@
 @import 'mixins';
 @import 'variables';
+@import 'variables-glitch';
 @import 'fonts/roboto';
 @import 'fonts/roboto-mono';
 @import 'fonts/montserrat';