pre-requisite course feature via milestones app

added edx-milestones repo

split milestones and pre-requisite course flags

added a unit test in lmd and fixed broken tests

removed extra line

updates to use milestone app api instead of signals

fixed broken test

making pre-requisite ddl property of settings page

user dashboard messaging

changes after marco's feedback

removed text-truncated from mixing and fixed broken test

removed pre-requisite gating logic

added lms boo choy test

UX changes after merge with master

fixed broken unit test