fix i18n scoping regression in layout files, fixes CNVS-14892

also simplify when/how we set the i18n scope... just hook into
Template#render, rather than the various renderers

test plan:
1. run canvas with RAILS_LOAD_ALL_LOCALES=true
2. change your language from english
3. go to any page in canvas
4. confirm the top nav and footer are translated
5. confirm that i18n is working elsewhere (see g/39399 for some good stuff
   to check)
6. note: if you're regression testing i18n everywhere, you'll also need to
   compile assets and run canvas with USE_OPTIMIZED_JS=true to get
   translations working in javascript land. that's technically not needed
   to verify this commit, but it'll eliminate any false positives

Change-Id: I08bc63f3bfc85e88369468f046a06b36811303b9
Reviewed-on: https://gerrit.instructure.com/39499
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>