Add feature toggles through Flipper

Gemfile

Gemfile.lock

changelogs/unreleased/feature-flags-flipper.yml

db/migrate/20170525174156_create_feature_tables.rb

db/schema.rb

doc/api/features.md

doc/development/README.md

doc/development/feature_flags.md

lib/api/api.rb

lib/api/entities.rb

lib/api/features.rb

lib/feature.rb

lib/gitlab/gitaly_client.rb

spec/lib/feature_spec.rb

spec/lib/gitlab/gitaly_client_spec.rb

spec/requests/api/features_spec.rb

spec/support/gitaly.rb