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

wait for course section to loaded

fixed broken boo choy test

changes after feedback