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/glitch/locales/en.json

@@ -29,5 +29,14 @@
   "settings.navbar_under": "Navbar at the bottom (Mobile only)",
   "status.collapse": "Collapse",
   "status.uncollapse": "Uncollapse",
-  "notification.markForDeletion": "Mark for deletion"
+
+  "notification.markForDeletion": "Mark for deletion",
+  "notifications.clear": "Clear all my notifications",
+  "notifications.marked_clear_confirmation": "Are you sure you want to permanently clear all selected notifications?",
+  "notifications.marked_clear": "Clear selected notifications",
+
+  "notification_purge.btn_all": "Select\nall",
+  "notification_purge.btn_none": "Select\nnone",
+  "notification_purge.btn_invert": "Invert\nselection",
+  "notification_purge.btn_apply": "Clear\nselected"
 }