fixes #13244 - update Rails to 4.2.6

- Add responders gem to support class-level respond_to usage
- http://edgeguides.rubyonrails.org/4_2_release_notes.html#respond-with-class-level-respond-to
- Replace foreigner with native Rails 4.2 FK support
- uses a patch from Rails 5 to support Foreigner's...

fixes #14638 - Refactor Subnet into STI to allow different subnet types

Fixes #14645: fix production images for plugins.

Images were going through the legacy asset helpers in
sprockets-rails, this commit sets some options on the context
class to ensure we are looking up digested images in the correct
path.

http://projects.theforeman.org/issues/14645

Fixes #12318 - support preview for deb-based distros

Fixes #14508 - foreman-rake console can take arguments

One can now say e.g.

foreman-rake console -- --sandbox

We need the --, otherwise foreman-rake itself tries to interpret the flags

Fixes #14479 - removed unwanted check for token param

fixes #9782 - implement mediapath for suse/yast

fixes #12962 - Add Xenserver to list of supported OS families.

Templates are included in community-templates repo.

Fixes #14232 - make generic helpers available for global PXE template

Fixes #14221: Use Sass Rails template when compiling plugin assets

The Sass::Rails::Template is needed to resolve css files imported
in an SCSS file without setting them to css imports which lead
to 404s in production.