Torznab API implementation for Sonarr/Radarr support.

[sergiotapia]

This PR does a few things:

• It updates the seeds for mix ecto.reset so Category records have torznab categories set for devs.
• It updates the db_seeder so new users on Docker get the torznab categories set.
• It creates a migration to set the torznab categories on existing Category records so existing users don't have to recreate their running instance's database.
• Finally, it creates an endpoint that returns XML in Torznab format with the right categories and fields and such.

I still need to point my Sonarr/Radarr to this endpoint to try it out, will do that next to verify if this actually works. If all goes well no more code changes will be necessary and this will Just Work (tm).

[dreulavelle]

Has support picked up for this feature? 👀