make routes work for media downloads

closes FOO-1186
flag=none

discussion entries and announcements
were generating a lot of failures

TEST PLAN:
  1) make a new discussion entry with a video
     embed.
  2) notification should get sent, and it should
     have a media download url pointing at the
     course the topic lives in

Change-Id: I888a8508bd18ac9e37727eab5ede9e7a0102f184
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/252278
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

lib/api/html/url_proxy.rb

@@ -91,11 +91,24 @@ module Api
       end
 
       def media_context
+        # not all objects are clean to find media urls for.
+        # If the current context is one of those classes,
+        # then we should look up what the actual context is that
+        # would have a reasonable media url on it (usually a course).
+        # If you are trying to do "media_redirect_url" on a context
+        # where you're finding a media url helper doesn't exist, you probably
+        # need to add a case here.
         case context
-        when Group
+        when Announcement
           context.context
         when CourseSection
           context.course
+        when DiscussionTopic
+          context.course
+        when DiscussionEntry
+          context.discussion_topic.course
+        when Group
+          context.context
         else
           context
         end

spec/lib/api/html/url_proxy_spec.rb

@@ -43,6 +43,25 @@ module Api
         it "passes through polymorphic urls" do
           expect(proxy.media_redirect_url("123", "video")).to eq("http://example.com/courses/1/media_download?entryId=123&media_type=video&redirect=1")
         end
+
+        it "has media redirect routes for discussion entries" do
+          course = course_model
+          topic = course.discussion_topics.create!
+          entry = DiscussionEntry.new
+          entry.id = 1
+          entry.discussion_topic = topic
+          proxy = UrlProxy.new(StubUrlHelper.new, entry, "example.com", "http")
+          expect(proxy.media_redirect_url("123", "video")).to eq("http://example.com/courses/#{course.id}/media_download?entryId=123&media_type=video&redirect=1")
+        end
+
+        it "can produce a redirect route for announcements" do
+          course = course_model
+          announcement = Announcement.new
+          announcement.id = 1
+          announcement.context = course
+          proxy = UrlProxy.new(StubUrlHelper.new, announcement, "example.com", "http")
+          expect(proxy.media_redirect_url("123", "video")).to eq("http://example.com/courses/#{course.id}/media_download?entryId=123&media_type=video&redirect=1")
+        end
       end
 
       describe "#api_endpoint_info" do