[Webhooks] Migrate Trigger section to Vue
A sub issue of #513631 , in order to keep MRs small.
This issue is for refactoring the Trigger section which needs: to be moved from app/views/shared/web_hooks/_form.html.haml to app/assets/javascripts/webhooks/components/webhook_form_app.vue
Design
#513631[Project_webhooks_settings_add_new_form_advanced_expanded.png]
![#513631[Project_webhooks_settings_add_new_form_advanced_expanded.png]](https://gitlab.com/gitlab-org/gitlab/-/issues/513631/designs/Project_webhooks_settings_add_new_form_advanced_expanded.png){kind=link}
Edited by Oiza Baiye