Modal > Add QA selectors

We can't currently check that a modal is shown or an action within the modal is visible within our QA tests. This means that if we have an important modal we want to validate against (a delete modal for instance), we can't do so.

We should add four different QA selectors to GlModal which will allow us to check each actionable part of the modal within QA tests:

Action	Selector
Modal container	qa-gl-modal
Modal cancel button	qa-gl-modal-cancel-btn
Modal primary button	qa-gl-modal-primary-btn
Modal secondary button	qa-gl-modal-secondary-btn