Add workhorse controller and API helpers

app/controllers/application_controller.rb

app/controllers/projects/avatars_controller.rb

app/controllers/projects/raw_controller.rb

app/controllers/projects/repositories_controller.rb

app/helpers/workhorse_helper.rb

db/schema.rb

lib/api/helpers.rb

lib/api/repositories.rb

lib/gitlab/workhorse.rb

spec/controllers/projects/raw_controller_spec.rb

spec/controllers/projects/repositories_controller_spec.rb

spec/lib/gitlab/workhorse_spec.rb