only display instfs pixel on enabled accounts

fixes RECNVS-319

test-plan:
- have inst-fs configured with REQUIRE_AUTH=true
- have canvas _configured_ for inst-fs, but inst-fs not enabled
- in safari:
  - delete any inst-fs cookies and log out of canvas
  - log in
    - inst-fs login pixel should not be displayed (verify with network
      request log in browser)
    - no inst-fs cookie should have been created
  - enable inst-fs
  - refresh the page
    - inst-fs login pixel should be displayed
    - inst-fs cookie should have been created

Change-Id: Idecb926e8f17b5c29948b38fd8b5afc6c298d78a
Reviewed-on: https://gerrit.instructure.com/140938
Reviewed-by: Andrew Huff <[email protected]>
Tested-by: Jenkins
QA-Review: Collin Parrish <[email protected]>
Product-Review: Cody Cutrer <[email protected]>

lib/inst_fs.rb

@@ -22,7 +22,7 @@ def enabled?
     end
 
     def login_pixel(user, session)
-      if !session[:shown_instfs_pixel] && user && app_host
+      if !session[:shown_instfs_pixel] && user && enabled?
         session[:shown_instfs_pixel] = true
         pixel_url = login_pixel_url(token: session_jwt(user))
         %Q(<img src="#{pixel_url}" alt="" role="presentation" />).html_safe