removed src/sass/neat as submodule

[#43606125]

It is versioned and installed by using the Ruby Gem
distribution now.
Include neat grid-layout extension as generated files

[#43606125]

neat grid-layout is a grid layout extension to sass/bourbon.

Previously it was included in the project as a git submodule.

Now the versioning is handled in the Gemfile.

When the neat Ruby Gem is updated:

  cd src/sass
  rm -rf neat
  ../../bin/neat install
Gemfile: updates to guard and related gems