Add admin UI to Restore/Confirm moderation of notifications

Add the admin UI needed to restore the hidden proposal notifications
when hidden by moderator. The admin can restore them or confirm
the moderation made by moderator, just like proposals.

config/locales/en/admin.yml

@@ -940,6 +940,15 @@ en:
           without_confirmed_hide: Pending
         title: Hidden proposals
         no_hidden_proposals: There is no hidden proposals.
+    proposal_notifications:
+      index:
+        filter: Filter
+        filters:
+          all: All
+          with_confirmed_hide: Confirmed
+          without_confirmed_hide: Pending
+        title: Hidden notifications
+        no_hidden_proposals: There is no hidden notifications.
     settings:
       flash:
         updated: Value updated