Expose CI enable option in project features

app/controllers/projects/application_controller.rb

app/controllers/projects_controller.rb

app/helpers/projects_helper.rb

app/models/project.rb

app/services/git_push_service.rb

app/services/projects/fork_service.rb

app/views/layouts/nav/_project_settings.html.haml

app/views/projects/edit.html.haml

app/views/projects/graphs/_head.html.haml

config/gitlab.yml.example

config/initializers/1_settings.rb

doc/api/projects.md

lib/api/entities.rb

lib/api/projects.rb

spec/factories/ci/projects.rb

spec/models/project_spec.rb

spec/requests/api/projects_spec.rb

spec/requests/api/services_spec.rb

spec/services/projects/create_service_spec.rb

spec/services/projects/fork_service_spec.rb