Rename resource service

[jensljungblad]

Consider naming Resource::Service something else. It was always a bit of a last resort. The name also leads to confusion with regards to other types of service objects in apps.

Perhaps we can go with Dashboard, which is the name used by https://github.com/thoughtbot/administrate. People are already familiar with the term.

class ArticleDashboard
  include Godmin::Dashboard
end