Refactor play_or_download() episode action code.
Streaming was always possible (#867) for all audio and video episodes if
an audio player was configured. Now it is only possible if a player is
configured for the episode's type.
Methods have been added to PodcastEpisode that are either now used by
actions or replicate the conditions used by actions to filter
selections. This will help prevent mismatched conditions between
play_or_download() and episode actions. And play_or_download() is easier
to reason about and should prevent future bugs such as #1250.
Fixes #1250.