Bug 875342: fix JS channel-specific features to use the right build-time logic, r=sstangl

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extra : rebase_source : 4d091d89ad5dd19ca0c966140a9a56041eced86d
Bug 486660: disable whatsnew and firstrun pages for Nightly builds, since we have no useful content for them, r=dolske

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extra : rebase_source : b8d7a51725255472812b3c03045419aa018a178a
Bug 840692 - Use HTTPS instead of HTTP for startup pages (startup.homepage_override_url and startup.homepage_welcome_url) r=gavin

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extra : rebase_source : 77a317d7aa5e01aa5b2b744d147a1ef2c076cc5c