Merge: nitweb: add `code` tab for modules, classdefs and propdefs.

Interesting demos:
* http://nitweb.moz-code.org/module/array_debug::array_debug
* http://nitweb.moz-code.org/classdef/core$Array
* http://nitweb.moz-code.org/propdef/core::abstract_text$Object$to_s

Only the two last commits are relevant since the rest belongs to #2170

Pull-Request: #2171
Reviewed-by: Jean Privat <jean@pryen.org>