Componentize metrics alerts modal form

Inside ee/app/assets/javascripts/monitoring/components/alert_widget_form.vue:

1. change <button>s to <gl-button>s
2. change <input>s to <gl-input>s
3. change .form-group div to <form-group />
4. ensure both Metric and Operator fields have labels

The following discussion from !14760 (merged) should be addressed:

• @jivanvl started a discussion: (+1 comment)

  How do you feel about changing these to the equivalent gitlab-ui gl-button?

  If you think this is out of scope, we can open an issue and discuss it there