Preparing a better endpoint for wardrobe-2020 to use! I deleted the
now-unused swf_assets#index endpoint, and replaced it with an
"appearances" concept that isn't exactly reflected in the database
models but is a _lot_ easier for clients to work with imo.
Note that this was a big part of the motivation for the recent
`manifest_url` work—in this draft, I'm probably gonna have the client
request the manifest, rather than use impress-2020's trick of caching
it in the database! There's a bit of a perf penalty, but I think that's
a simpler starting point, and I have a hunch I'll be able to make up
the perf difference once we have the impress-media-server managing more
of these responsibilities.
