[WIP] Port site to nanoc 4.x

**This is a work in progress** branch and should remain un-merged until nanoc 4.0 is released.** This pull request is meant to collect changes I had to make to the web site code to make it work with nanoc 4.0.

Changes:

* Added used nanoc gems to _Gemfile_ and made them required in _lib/filters.rb_ and _lib/helpers.rb_
* Got rid of static data source and move everything into _content/_
* Merged compilation and routing rules
* Made some changes specific to the nanoc site, e.g. replace `Site::DEFAULT_CONFIG`
* Removed the use of `#children`
* Made my own `:pre` snapshot (which I now call `:before_layout`)
* Relaxed the “title does not match latest release info regex” error into a warning
* Added a bunch of monkey patches to nanoc or dependent libraries in _lib/fixes.rb_