FEATURE: Actually show more notifications

The "Show more notifications..." link in the notifications dropdown now
links to /my/notifications, which is a historical view of all
notifications you have recieved.

Notification history is loaded in blocks of 60 at a time.

Admins can see others' notification history. (This was requested for
'debugging purposes', though that's what impersonation is for, IMO.)

app/serializers/user_serializer.rb

@@ -43,6 +43,7 @@ class UserSerializer < BasicUserSerializer
              :suspended_till,
              :uploaded_avatar_id,
              :badge_count,
+             :unread_notification_count,
              :has_title_badges,
              :edit_history_public,
              :custom_fields
@@ -76,6 +77,7 @@ class UserSerializer < BasicUserSerializer
                      :tracked_category_ids,
                      :watched_category_ids,
                      :private_messages_stats,
+                     :unread_notification_count,
                      :disable_jump_reply,
                      :gravatar_avatar_upload_id,
                      :custom_avatar_upload_id,
@@ -242,6 +244,10 @@ class UserSerializer < BasicUserSerializer
     object.badges.where(allow_title: true).count > 0
   end
 
+  def unread_notification_count
+    Notification.where(user_id: object.id, read: false).count
+  end
+
   def include_edit_history_public?
     can_edit && !SiteSetting.edit_history_visible_to_public
   end