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

LMS-1296
Port spoc gradebook to new instructor dash

LMS-1296
Change URL patterns for instructor dashboard

Fix acceptance and unit tests

LMS-1296
Add segment.io tracking for legacy instructor dashboard

LMS-1296
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

LMS-1296
Only show banner on legacy dash for edx.org

LMS-1296
Merge pull request #2865 from edx/sarina/change-default-dashboard

WIP Switch default instructor dashboard to new dash