Change API to automatically use plural types

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I tried to use a new feature of JaSerializer where it can generate plural types by default but got bogged down in mysterious seemingly-unrelated incompatibilities because of a new dependency on plug_cowboy. This is only happening because I bypassed Ember Data in the overlaps-checking and Mirage serialises with plural types but the API has singular types, which results in some ugly type-checking when sifting results. For now I’m manually overriding the relevant types (backspace/prison-rideshare-api@97982d9) but it would be nice to use this feature. Maybe I’ll have to update to Phoenix 1.4 or something…?