Page contents now sent as Wikitext

We now do the rendering client-side!  Very neat.

Moved the Wikitext class hierarchy into shared.  Had to make a few
tweaks to make it pickle and unpickle properly.  Changed the actual name
of the HtmlWikitext case class, because having multiple constructors
confuses upickle.  Also, made it not use Play's HTML class directly,
because that can't be pickled.