Implement support for some basic embed options for work sheets via LTI

This commit also fixes an issue with the flash messages being positioned too high and displayed for too long

spec/controllers/submissions_controller_spec.rb

@@ -186,7 +186,7 @@ describe SubmissionsController do
         end
 
         it 'ends with a placeholder' do
-          expect(url).to end_with(Submission::FILENAME_URL_PLACEHOLDER)
+          expect(url).to end_with(Submission::FILENAME_URL_PLACEHOLDER + '.json')
         end
       end
     end
@@ -196,7 +196,7 @@ describe SubmissionsController do
         let(:url) { JSON.parse(response.body).with_indifferent_access.fetch("#{action}_url") }
 
         it "corresponds to the #{action} path" do
-          expect(url).to eq(Rails.application.routes.url_helpers.send(:"#{action}_submission_path", submission))
+          expect(url).to eq(Rails.application.routes.url_helpers.send(:"#{action}_submission_path", submission, format: :json))
         end
       end
     end