Wait for polyfiller to load before booting front end
Wait for polyfiller to load before booting front end

Fixes FOO-3043
flag=none

Recent refactoring of the front-end capability loading
introduced an error where the completion of the I18n
capability was not waiting on locale data to actually
finish fetching before marking the capability as fully
up. Thus the front-end would continue booting since it
thought all the dependencies were loaded. And if the
locale polyfiller had not actually completed yet, the
first use of, say, Intl.NumberFormat() could explode.

Test plan
* Set your locale to a polyfilled one (Cymraeg, Íslenska,
  or Nynorsk are good ones)
* Try to create a new assignment in a course by visiting
  /courses/:id/assignments/new  (where :id is the id of
  a course you have created)
* The "polyfilled XX for locale XX" messages should show
  up in the console indicating the the locale data is
  being polyfilled
* The page should also load and function normally and not
  throw any fatal errors

Change-Id: I7a7710c626906f43027f192eca70a9b56ed33554
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/298079
Reviewed-by: Jacob Burroughs <8ecea6e385af5cf9f53123f5ca17fb5fd6a6d4b2@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Charley Kline <05950d7e90ba5e3da323378368c22b3ffaaf6647@instructure.com>
Product-Review: Charley Kline <05950d7e90ba5e3da323378368c22b3ffaaf6647@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms