Separate Rails additions to bbq-rails

[mostlyobvious]

Things like:

• https://github.com/drugpl/bbq/blob/master/lib/bbq/rails/routes.rb
• https://github.com/drugpl/bbq/blob/master/lib/bbq/railtie.rb

are useful but totally not essential to core. This would help to resolve dependency problems as well and move responsibility to test compatibility with Rails to that gem.