Update doc string for YouTube stub server.

Included information on how to launch the server
on its own.
EOF
Add an option to prevent multiple logins of the same user.
Merge pull request #2793 from edx/dcs/mat-17

Add an option to prevent multiple logins of the same user.
Fix broken test due to refactoring.
LMS-1287: Added section for batch add of beta testers
Merge pull request #3071 from edx/ahodges/documentation/bulkbeta

LMS-1287: Added section for batch add of beta testers
Merge pull request #3090 from edx/valera/update_youtube_stub_server_doc

Update doc string for YouTube stub server.
Make YouTube mocked server serve YouTube API

1.) Making YouTube mock server serve YouTube API.
2.) Adding ability to block YouTube API from YouTube mock server.
3.) Adding acceptance tests that check YouTube mock server for availability of YouTube API, and for it's ability to block YouTube API.

Part of BLD-939.
Addressing comments.
Addressing comments.