Make MWTransclusionModel and MWTemplateDialog extensible

Wikia has done some work on the template user experience, including
automatically showing all available parameters without the use of
TemplateData. In order to make our changes, we had to make some changes
to VE-MW.

ve.dm.MWTransclusionModel.js
* this.specCache is created so subclasses can reference it.
* Promise handlers in the fetch() method have been broken out as class
  methods so they can be overridden in subclasses.

ve.ui.MWTemplateDialog.js
* addPromptedParameters() has been moved to the
  initializeNewTemplateParameters() class method so subclasses can
  overwrite. In Wikia's implementation, we have a method of getting
  all parameters and a dialog that shows all of the parameters, so the
  request to addPromptedParameters is overwritten.
* Added a done() handler to the transclusionModel promise for Wikia
  extensibility.

Change-Id: I073c5850420e7719e82957f879423c2717af674a
Merge "Make MWTransclusionModel and MWTemplateDialog extensible"