Rename layouts/base to layouts/layout (as per conventions)

[tnir]

In #748, you need to set set :layout, :base in the config, but Middleman has the default filename source/layouts/layout.*. To follow the conventions, rename source/layouts/base.haml to source/layouts/layout.haml with updating references in source/layouts/* files.

This PR focuses on removal of source/layouts/layout.haml, which is the default filename. In addition, the most common layout in this project source/layouts/base.haml has been referred from customized layout files. In another place, we may change base to the default name.