Improve pipeline triggers UI

app/assets/stylesheets/pages/settings_ci_cd.scss

app/controllers/projects/triggers_controller.rb

app/models/ci/trigger.rb

app/policies/ci/trigger_policy.rb

app/views/projects/triggers/_content.html.haml

app/views/projects/triggers/_form.html.haml

app/views/projects/triggers/_index.html.haml

app/views/projects/triggers/_trigger.html.haml

app/views/projects/triggers/edit.html.haml

changelogs/unreleased/add-pipeline-triggers.yml

config/routes/project.rb

spec/features/triggers_spec.rb

spec/models/ci/trigger_spec.rb

spec/policies/ci/trigger_policy_spec.rb