[videodb] fix seasons update query for mysql, which doesn't like SELECT subqueries on the same table
[cleanup] followup to f86b9d2884
[videodb] fix tvshow view for mysql by utilising a subview. Thanks to @sccplg
Merge pull request #4988 from mkortstiege/threadmsgs

[cleanup] followup to f86b9d2884
Merge pull request #4985 from jmarshallnz/fix_mysql_query

[videodb] fix seasons update query for mysql