Don't try to import accountlinker accounts if channel_id is not set

When we have run the command to lookup the channel ids, it fails for
many of the rows in our database.  I believe this is because the user
has either revoked access to amara, deleted/altered their account, or
done something similar.  We should just forget about the account and
ask the user to re-link it if they want to.
Fixes for creating youtube feeds

When creating them for users, correctly set the user field

Handle an existing feed for the same URL:
  - If the feed is owned by the same user/team as the youtube account,
    then link it to the account
  - If it's owned by a different user/team, then throw an error
    (if this happens in practice we'll need to handle it).
Schedule youtube feed updates immediately when linking an account
Merge pull request #1659 from pculture/youtube-tweaks

Youtube tweaks