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
Merge pull request #1567 from pculture/gh-1488-prep

Gh 1488 prep