Support `js-close` as a removal class in UserCallout

What does this MR do?

Add support for dismissing elements with the class .js-close when a User dismisses a callout.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

While working on https://gitlab.com/gitlab-org/gitlab-ee/issues/5710 I realized that UserSupport allows to remove elements with the .close class. The only problem is, that this class applies CSS styles as well. So I created support for a second class .js-close, which should has no styles applied to it.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • [n/a] Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• [n/a] Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/5710
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5601