give wiki-pages a media redirect path

refs FOO-1186
flag=none

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

Change-Id: I7b5b3599bf264b5b3aa0cc1c4bb4d09fc135b2cf
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/253934
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

lib/api/html/url_proxy.rb

@@ -109,6 +109,8 @@ module Api
           context.discussion_topic.course
         when Group
           context.context
+        when WikiPage
+          context.course
         else
           context
         end

spec/lib/api/html/url_proxy_spec.rb

@@ -54,6 +54,13 @@ module Api
           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 "has media redirect for wiki pages" do
+          course = course_model
+          page = course.wiki_pages.create(:title => "some page")
+          proxy = UrlProxy.new(StubUrlHelper.new, page, "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