Use JsObject for serialising item meta in Facia Press (22 field limit on case class!)

Boring reasons for why there is a 22 field limit: http://www.scala-lang.org/old/node/7910

The short of it is we ought to fix this by making our data-types nicer. Fields like all the image cut out ones should be in their own data type, so instead of occupying four fields, it only occupies one. That's not a simple change at this point, and as throughout the rest of the stack we treat the metadata as an untyped map, we'll revert to doing so here as well.

CC @janua @stephanfowler