Implement ve.dm.MWTemplateNode.getWikitext()

Escapes }} and | by wrapping them in <nowiki> tags, and steps around
<nowiki> tags so as to not double-wrap things.

Change-Id: Ia50906524c1fcf55c9f390a114856bc7f20b2d3a
Merge "Implement ve.dm.MWTemplateNode.getWikitext()"