Cache static assets using an etag header.

When Pilcrow returns static assets, it now sets an `ETag` header, derived from the content of the asset. This header will change if the asset changes.

Conforming browsers _may_ cache the response, and make a conditional request with an `If-None-Match` header on future requests for the same asset. If we see that header, Pilcrow will check whether the loaded asset is the same as the one the browser already had, and skip the response with a 304 if appropriate. This cuts down on the number of times clients will load the same script files and the same assets from the server.

Endpoints that route to the index after doing some logic got a pretty major cleanup. The tangled logic between the actual handler and the error type made them challening to follow, and there wasn't a clean way to pass the `If-None-Match` header through into the error for use when determining the final response. This version instead combines the negative cases with _success_, which produces the desired responses with much more straightforwards code.

I've also opted to support `If-None-Match` for these endpoints, even though they do logically change if the underlying chat state changes - because the _response body_ does not change, and that's what the HTTP spec (and HTTP clients) care about in this context. They will, however, return the response in full for situations like Not Found.

(I know it looks like these endpoints now _require_ the `If-None-Match` header. Trust me on this: they do not. The `headers` create cooks up an empty `If-None-Match` header if none is supplied, and that empty header differs from any non-empty ETag.)

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