Feature: Store anonymised totalled counts of listening and downloading

[pvj]

Having a number of listeners and downloads for stored episodes and shows can be useful internally and externally both

In our schema we already have download_count we don't use it however. On each successful request to the episode download endpoint we should increment this value and commit it to db

We need to expand our schema and introduce listening_count which I think is missing.

These both should be stored as integer

download_count & listening_count should both be added to our serialisation dumps for episodes.

We might introduce a class method on shows objects that collect the counts of all episodes hosted by the show. In that case that is not part of the model as we expect that adding all selected rows would not be a huge performance hit. If it is we should expand the show schema.

In either case show-serialisation has to include the totalled counts.