First pass generating jsduck docs for Parsoid + some dead code removed

* Write documentation, clean it up, standardize it.
  Generate docs by going to js/ and running jsduck --config=jsduck-conf.json
  (you'll need jsduck, which is a ruby gem)

  Note that I haven't totally finished documenting everything, and some of
  the things documented in this patch just have some skeleton comments.
  We'll improve upon the documentation incrementally.

  Current status:
  - No jsduck warnings.
  - Several very widely-used classes documented.
  - Many less-used but frequently-changed classes documented.
  - A lot of dense code documented and now readable in a hypertext format
  - Some private methods converted to use "_" prefix within the class.
  - Not all private methods have been marked @private, and not all of them
    use the "_" prefix yet.

* Non-doc fixes:
  - ParsoidConfig.prototype.traceFlags is now null by default.
  - Deleted the unused LinearModelConverter class.
  - Deleted dead code from mediawiki.parser.environment.js
  - Removed unnecessary normalizeTitle calls in ext.core.LinkHandler.js

* No change in parser test results.

Change-Id: I4f20fc7c083f661dd71d582e89c48229f4bd6dc4