Refactor HTML attribute preservation

Rather than using namespaced linmod attributes, store the preserved
HTML attributes in the .htmlAttributes property of the linear model
element, in a nested structure to allow for easier treatment of child
nodes. Also added attribute order preservation by storing attributes
as an object plus an array of keys.

ve.ce.Node.js:
* Remove html/* attribute synchronization. Doesn't make sense any more
  because these things aren't in the attributes object any more. I don't
  think it ever made sense because these attributes were never supposed
  to be changed anyway.

ve.ce.View.js:
* Replace renderAttributes() with a simple wrapper around
  renderHtmlAttributeList()

ve.dm.Converter.js:
* Add buildHtmlAttributeList() and renderHtmlAttributes() for building
  and rendering HTML attribute lists

ve.dm.Model.js:
* Add getter for .htmlAttributes

ve.dm.Node.js:
* Drop .htmlAttributes on clone, and remove logic dropping html/*

ve.ui.MWCategoryWidget.js:
* Remove html/0/about hack, was already unnecessary and now doesn't
  work any more

tests/:
* UPDATE ALL THE TESTS

Change-Id: I620573afd70d36ade6b80413075b6e1f4a435abe
Merge "Refactor HTML attribute preservation"