Add JSONContent and handler from EventLogging

As was discussed at the architecture summit, a basic JSON content
class which handles validation and basic display. Not intended to
be used directly, but for extensions to subclass.

Co-Authored-By: addshore <addshorewiki@gmail.com>
Change-Id: Ifcde9bcd0efcf15a3ab692dd2a0a3038559e0254