Bug 949906 - Add a callback to modify evaluated moz.build sandboxes, fix Sphinx docs; r=glandium

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extra : rebase_source : 9ef2219145fb754a9cbe9e7e30b6f2841910f13f
Bug 928195 - Part 6: Add docs for WebIDL and the build system; r=froydnj

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extra : rebase_source : cee29c0bf5c050d73ef02d6ad933802b8fad21e0