Incrementing media counter at source level mapped to episode counter in file format

[meeb]

Add a "number of media items" counter to the source model, and set it at the media item level as an immutable "media position in source" index. This is to be used as a hacky "episode" counter and needs to be presented as a {episode} format variable for file name formatting.

This counter needs to ensure uniqueness so old media is deleted and new media added it continues to increment.

This will be a mostly arbitrary number, but it will allow people to address media files with a unique "episode number" per source, even if the episode number is effectively just the order in which the media item was added to TubeSync for a source.