rename `Mime` to `MediaType`

Tracking issue: #274

I saw akka refers to this as [`MediaType`](https://doc.akka.io/api/akka-http/10.0/akka/http/scaladsl/model/MediaTypes.html) and quite liked it.

"MIME" is pretty nondescript if you don't know what it is I'm advance. "Media type" seems a bit clearer and in turn more natural to talk about:

> "In order to get the media type, call the content_type method on the request"

For search purposes we should probably still mention "MIME" in the docs somewhere, but the type itself being called `MediaType` would be nice I think.