don't localized datetimes formatted for datepicker

refs CNVS-12398

the eventual complete solution will involve teaching the date picker to
parse the localized datetime, but for now, just format the time in
english if the target is a datepicker's field value.

test-plan:
 - set locale to portuguese
 - datepickers (e.g. assignment due date when editing) populated on page
   load should contain the date time in english (e.g. "<date> at 3:00pm"
   vs. "<date> em 3:00")
 - using the date picker should produce a time in english in the field
 - saving the english time should work
 - non-datepicker field values with the same formatting rules (e.g.
   assignment due date when *not* editing) should display localized

Change-Id: Ibc69d2066518b4875c1f42b794df784c7e70c2c7
Reviewed-on: https://gerrit.instructure.com/33444
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>