don't allow locale names or codes to be overridden, fixes CNVS-14465

basically two changes:
1. ensure locales.yml is loaded last, to ensure we don't get any wonky
   overrides from translators
2. don't send those strings off for translation any more

test plan:
1. in a rails console w/ RAILS_LOAD_ALL_LOCALES=true, confirm that:
   >  I18n.backend.direct_lookup("nl", "qualified_locale")
  => "nl-NL"`
2. run `rake i18n:generate` and confirm that "qualified_locale" does
   not appear anywhere in ./config/locales/generated/en.yml
3. in canvas (w/ RAILS_LOAD_ALL_LOCALES=true), change your locale to
   "Nederlands (Crowd Sourced)"
4. confirm that you can still view wiki pages

Change-Id: I3923f58d7b9433a9e5e062e16e1556d79f845ae4
Reviewed-on: https://gerrit.instructure.com/38475
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>