transcript and chapters for episodes should allow external resources

Is your feature request related to a problem? Please describe

Currently transcript_uri and chapters_uriin cp_episodes can only point to a resource stored locally in public/media/podcasts/ folder.
In order to allow external services (such as Hypercatcher), Castopod should provide a way to choose between a local file and an external URL.

Describe the solution you'd like

No idea.

Describe alternatives you've considered

None.

Additional context

See #110 (closed) for Hypercatcher chapter management.