point subtitle form to amara.org not universalsubtitles.org

universalsubtitles.org returns a 503 error now

fixes: CNVS-14832

test plan:

try to make subtitles for a video, 
make sure the page it takes you to exists.

Change-Id: I162247580f6a30f530725725c657dfc45ee4e3b7
Reviewed-on: https://gerrit.instructure.com/39396
Reviewed-by: Paul Hinze <paulh@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>