Language selection label - "English" is now "English (US)"

Making it known that the default english locale is en_US

Fixes INTL-2

test plan:
- open up user settings
- click edit
- open the language dropdown
- make sure that "English" no longer shows up while "English (US)" does
- save your changes
- validate that the changes were made
- this will affect any place that renders the local name from i18n.available_locales

Change-Id: Ice1f00655be1f83293553e1875ab377ad9008f1a
Reviewed-on: https://gerrit.instructure.com/38281
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>