properly order new announcements. fixes CNVS-3242

maintain a separate position scope for announcements than
discussions so that new announcements receive proper
position value.

test plan:
  * create multiple announcements in a course, verifying
    that the newest announcements appear at the top of the
    announcement index page.

Change-Id: Ie05d6399b670326e02a0634598e924f89e92eeea
Reviewed-on: https://gerrit.instructure.com/21761
Reviewed-by: Mark Ericksen <marke@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cam Theriault <cam@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>

Author: zachpendleton

app/models/announcement.rb

@@ -30,7 +30,11 @@ class Announcement < DiscussionTopic
   validates_presence_of :context_id
   validates_presence_of :context_type
   validates_presence_of :message
-  
+
+  acts_as_list scope: %q{context_id = '#{context_id}' AND
+                         context_type = '#{context_type}' AND
+                         type = 'Announcement'}
+
   def infer_content
     self.title ||= t(:no_title, "No Title")
   end