respect locale preferences on new user account confirmation

fixes CAT-399

Test Plan
  1. Fire up canvas with full i18n support configured
     (i.e. $ RAILS_LOAD_ALL_LOCALES=true rails server)
  2. Submit a POST request to the canvas api to create a new user
     (i.e. '/accounts/:id/users') with request headers [:user][:locale]
     set to a supported canvas locale (i.e. 'zh')
  3. Navigate to the new user account confirmation page from the email
     link and ensure the page is rendered with localized content

Change-Id: I85f21864b03086af922d41a726a0547155248eb8
Reviewed-on: https://gerrit.instructure.com/39204
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>