Backend for a gitignores dropdown

app/helpers/blob_helper.rb

app/models/repository.rb

app/views/projects/blob/_editor.html.haml

app/views/projects/empty.html.haml

lib/api/api.rb

lib/api/entities.rb

lib/api/gitignores.rb

lib/gitlab/gitignore.rb

lib/tasks/gitlab/update_gitignore.rake

spec/lib/gitlab/gitignore_spec.rb

spec/requests/api/gitignores_spec.rb