Rewrite multipart middleware to parse multiparts with content-type

[andrewmcveigh]

This PR fixes a number of issues with the previous multipart handling implementation:

• Swagger UI updated to latest release, so that content-type multipart header is passed in the request
• Re-implemented ring multipart middleware to not throw away content-type information in string body multiparts
• Decode multiparts using reitit configured content decoders
• Add ld+json muuntaja decoder to /data endpoints so that it's properly configured
• Properly handle a couple of multipart parsing edge cases, exposed by swagger-ui's update

[RickMoynihan]

I don't think we need this work at the minute, though we should keep the branch around as it's useful to have, and there's a chance we might need it later.

Closing for now.