Deal better with non-supported videos
Added migration/management command to lookup youtube channel ids

We want to migrate to the new externalsites code in the gh-1488
branch, however to do that we need to have youtube channel_ids instead
of usernames.  This branch can be used to generate those in
preparation of the migration.
Merge remote-tracking branch 'origin/staging' into gh-1488-prep
updated docker setup to use new closure path
Merge pull request #1567 from pculture/gh-1488-prep

Gh 1488 prep
Updated loading screen for embedder
Merge branch 'gh-1566' into gh-548
Adjust foreground color because of bg color change
Adjust info message
Merge pull request #1568 from pculture/gh-1566

Updated loading screen for embedder
Merge pull request #1570 from pculture/gh-548

Gh 548
Thumbnail mechanism in embedder
Added nice-looking play button
Do not show thumbnail container if there is no thumbnail
Point to correct unisubs-integration commit
Add a background color to video element in embedder so that it looks nicer for mp3s on Chrome
made play button slightly transparent
Merge pull request #1572 from pculture/gh-1533

Add a background color to video element in embedder so that it looks nic...
Merge pull request #1571 from pculture/gh-1525

Point to correct unisubs-integration commit
Fixed conflict in compiled CSS