Improve notifications page (#12497)

Currently notifications page seems a bit cluttered with no clear
separation between e-mail and filtering settings. This commit tries to
address them by adding clear separation with headers, hints and removing
continuously reused texts for events checkboxes.

config/locales/en.yml

@@ -886,6 +886,10 @@ en:
       body: 'Your status was boosted by %{name}:'
       subject: "%{name} boosted your status"
       title: New boost
+  notifications:
+    email_events: Events for e-mail notifications
+    email_events_hint: 'Select events that you want to receive notifications for:'
+    other_settings: Other notifications settings
   number:
     human:
       decimal_units: