Order by recently added on movie & series indexes.

It's a more useful default than alphabetical, until I get some kind of
sorting, filtering and searching UI stubbed out.

(Accidentally included the series part of this in the previous commit,
and had pushed before I noticed it. Ho hum.)
Use mtime in place of created_at for VideoFiles.

The file modification time is more useful than the real created_at,
particularly when creating DB records from an existing library of files.