Update doc string for YouTube stub server.

Included information on how to launch the server
on its own.
EOF
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
Support for reordering built-in-tabs.
Enable viewing of hide option for hideable pages.
Fixed quality and coverage errors, added tests.
removed debugger call.
added course_tab class when new tab is added.
fixed issue found by acceptance test - reordering newly added static tabs.
renamed css class "course_tab" to "course-tab".
changes per PR review:
  - remove analytics call
  - raise not implemented error for unsupported POST requests
Introduced new css class "sortable-tab" to prevent dragging tabs
into non-movable tabs.
Added acceptance tests for hiding wiki and reordering of tabs.
Added acceptance tests for new reordering of pages feature.
Modified get_tab_by_slug API to be consistent with other get_tab_by_X APIs.
Added additional documentation.
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
Merge with Brian's changes.
Dev code cleanup in html file.
Remove textbook tab if no more textbooks.
Changes to address PR comments.
Support for "View Live" on Pages page.
tabs.py: tab -> tab_dict
pages.py: rename CONSTANT, pylint fixes
PR review change: renamed variable.
Copied environment settings for ENABLE_STUDENT_NOTES and ENABLE_TEXTBOOK
from LMS to CMS.
Add code to not display empty collection tabs.  This was needed as
the default tabs includes the empty Textbooks tab.