Fix multiple videos on page

If there are several videos on page, make sure that
the global onYouTubeIframeAPIReady function is properly
setup so that all callbacks are trigerred when the YouTube API
loads.

BLD-972
Adding acceptance test for multiple videos.

Part of BLD-972.
Merge pull request #3124 from edx/valera/fix_multiple_videos_on_page_RELEASE

Valera/fix multiple videos on page release
Merge branch 'release'

Conflicts:
	lms/djangoapps/courseware/features/video.feature
Open ended panel headings not internationalized.
LMS-2169
Merge pull request #2940 from edx/waheed/lms2169-open-ended-panel-headings-not-internationalized

Internationalized open ended panel headings.
Fixed lagacy dash numeric placeholders with named.
raise ItemNotFoundError if course not found in modulestore (course_listing)
STUD-1455
Merge pull request #3086 from edx/waheed/fix-lagacy-dash-numeric-placeholders

Fixed legacy dash numeric placeholders with named.
Add acceptance test for blocked YouTueb API with HTML5 sources

When the video has both YouTube IDs and HTML5 sources, and
the YouTube API is blocked, then HTML5 sources should be
used.
Addressing comments.
Merge pull request #3010 from edx/zub/bugfix/std1455-studioaccessissue

raise ItemNotFoundError if course not found in modulestore (course_listing)
Merge pull request #3102 from edx/valera/add_acceptance_test_youtube_html5_blocked_api

Add acceptance test for blocked YouTueb API with HTML5 sources