Add new FileEpisode table to DynamicModel.

Add new FileEpisode table to DynamicModel.

This new table shows all files related to an episode
and not just the files for which an episode is the
main episode.

To get all episodes a UNION is needed over file and file
joined with file_episode_rel, but Dynamic Model's query
builder does not support building such queries. The
workaround is to do the UNION in a view and query it
instead.

File will be changed to what FileEpisode does in the
future as it was File's intended behaviour.