Rename ExtResponseRepresentation to ResponseRepresentation

The site-toolkit codebase contains a class ExtResponseRepresentation which is the response envelope for most of the REST calls.

Historically, this was indeed tightly coupled to ExtJS but that is no longer the case.

Subsequently, the targeting codebase contains a class ExtResponse, which is very similar to ExtResponseRepresentation (and has the same purpose) so we should drop it and just re-use ResponseRepresentation in targeting.