Migrate the user.subscribedPodcastIds column to a new table with a relationship to users

[mitchdowney]

Currently subscribedPodcastIds are saved as an id in an array in a json column on the User table. This is an overly simplistic implementation, and we could derive more benefits by migrating this to a separate table with a relationship to the User table.

This will require:

1. creating a new table in the database.
2. migrating existing data to the new table.
3. updating the API to return the new table data whenever subscribedPodcastIds was returned previously.

Hopefully updating the client-side will not be necessary, by updating the existing endpoints to return the data from the new database schema.

https://github.com/podverse/podverse-api/blob/develop/src/entities/user.ts#L148