Switch default instructor dashboard to new dash

LMS-1296
Add banners to the instructor dashboard

Notification banner to default dash to announce the change

Warning banner on legacy instruction dashboard to explain it has been deprecated
Port spoc gradebook to new instructor dash
Change URL patterns for instructor dashboard

Fix acceptance tests

Fix unit tests
Add segment.io tracking
UI cleanup of new instructor dashboard

Stylize warning banners; more UI touches for the intructor dash:
clean up nav structure, compact header so content is higher on the page
Only show banner on legacy dash for edx.org