Propshaft support

The rails_admin gem does not work in apps that use Propshaft to serve assets instead of Sprockets. I was able to work around it like this, but it would be nice to support Propshaft out of the box.

config/initializers/assets.rb:

# stub for rails_admin compat
class Sprockets
  def self.register_bundle_processor(...) = nil
end

Also, one needs to grab application.js, application.css, and the fa-solid woff2 font from a sprockets based rails_admin live page. Place in app/assets/stylesheets/rails_admin and app/assets/javascripts/rails_admin.

The CSS needs to be modified to change the fa-solid url to a propshaft compilable url (i.e, take out the digest).