i18n public course checkbox on create course form

fixes GH-470

Change-Id: I684a0d65e7423ea6c4e11b12de9639ff071540a5
Reviewed-on: https://gerrit.instructure.com/37516
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>