change the name for the add new app button, and update custom fields label

fixes PLAT-573

test-plan:
*The title of the Add New App modal should be "Add New App"
*Screen readers should read the extra help text for the Custom Fields text box

Change-Id: Icdd277542f8fddf9165f95f2add26bbab741c0ab
Reviewed-on: https://gerrit.instructure.com/38576
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>