manage focus to/from/within password reset modal appropriately

Shamelessly appropriates @antoviaque's work on https://github.com/edx/edx-platform/pull/902/ and applies it to the "forgot password" modal.

Now, when you click "forgot password", focus is shifted to the new modal's "x" button. Tabbing now appropriately cycles through the modal, and when the modal is exited, focus is returned to the "forgot modal" link.

@talbs @singingwolfboy 


(addresses https://edx-wiki.atlassian.net/browse/LMS-1166)