i18nliner-js (part I)

this swaps out our "parsing" with i18nliner-js

also remove localization abilities of I18n.beforeLabel, since it's not
worth the trouble to support. it can still be called to format a string.

notable changes/fixes in generated yml:
1. client_apps are picked up by i18n:rake so they'll get translated...
   due to the old short-circuiting logic (`rc = rc && ...`), it would
   stop processing scripts within a particular file after the first one
   it found without an I18n.t
2. we no longer incorrectly double-escape special chars in our js strings
   (e.g. newlines are actually newlines, not a literal "\n")

test plan:
1. verify string extraction:
   1. `rake js:generate i18n:generate` before and after this commit
   2. confirm `config/locales/generated/en.yml` is identical, except the
      notable changes/fixes listed above
2. verify js translation file generation:
   1. `rake i18n:generate_js` before and after this commit
   2. confirm the files in public/javascripts/translations are identical
3. verify client_app checker still works:
   1. `cd client_apps/canvas_quiz_statistics/`
   2. `grunt check_i18n`

Change-Id: Ic8ad058bee1c9476f42916f10b612c1c08863fe3
Reviewed-on: https://gerrit.instructure.com/42809
Reviewed-by: Michael Ziwisky <mziwisky@instructure.com>
Product-Review: Michael Ziwisky <mziwisky@instructure.com>
QA-Review: Michael Ziwisky <mziwisky@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>