Speedup for team video search index

Use a COUNT query, instead of calling len() for all_sls.

Only execute the query once for completed_sls.
Speedup SubtitleLanguage.having_non_empty_tip()

The INNER JOIN to a subquery wasn't being optimized very well by
mysql.  I think it was actually going through all subtitle version in
the DB.

Replaced it with a different subquery that returns the same results
and is optimized well by mysql.