Update doc string for YouTube stub server.

Included information on how to launch the server
on its own.
EOF
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
Reduce false cache misses when evaluating prerequisites
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.
Merge pull request #3085 from edx/valera/make_youtube_mocked_server_serve_youtube_api

Make YouTube mocked server serve YouTube API
Add the ability to reorder Pages and hide the Wiki page.
Studio - revising markup, templates and styling for page editing UI:

* added in fixed state markup/logic
* added in collection markup/logic
* revised styling for page titles, collection, and fixed elements
Support for "View Live" on Pages page.
Settings for ENABLE_STUDENT_NOTES and ENABLE_TEXTBOOK in CMS.
Do not display empty collection tabs.
Updated Changelog.
Merge pull request #3064 from edx/nimisha/studio-reorder-and-hide-tabs

Nimisha/studio reorder and hide tabs
Merge pull request #3097 from edx/dcs/paver-reqs

Reduce false cache misses when evaluating prerequisites
Watch for asset changes.