i18n developer key index and external tool edit

fixes GH-471
fixes GH-472

test plan:
- make sure "Add Developer Key" and table headers are localized on the
  developer keys page in site admin
- make sure "Name", "Consumer Key", "Shared Secret", "Configuration Type" are
  localized when editing an external tool configuration

Change-Id: I6ab2cc981fe296ba33f96589f805a2985d22da7d
Reviewed-on: https://gerrit.instructure.com/37551
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>