Rename `.../cast` to `.../characters`

[cmil]

The /corpora/{corpusname}/plays/{playname}/cast and /corpora/{corpusname}/plays/{playname}/cast/csv endpoints are kind of a misnomer since these lists do not really involve any actors or cast. I suggest to replace "cast" with "characters" in order to avoid misunderstandings.

[lehkost]

Thanks for bringing this up. I totally agree, especially because we also collect all speaking characters, even if they're not mentioned in the cast list.