Add support for editing multi-template nodes

Objectives:

* Refactor template dialog to support loading template data for, and
  editing multiple templates and interleaved content in a template node
* Update template node model to generate multi-template wikitext

Changes:

ve.dm.MWTemplateNode.js
* Rewrite getWikitext method to work with multi-template formats

ve.dm.MWTemplateDialog.js
* Retain information about the node and template calls
* Break AJAX handler into its own method
* Attach event handlers to inputs directly so template values are
  edited directly on the fly
* Refactor page building to support multiple templates
* Add multi-template support for template data API call
* Add support for editing plain text content

Change-Id: I92ff8a9e186701a3f8da88def92a5b7dcb607897