Add test for mentioning a user on an issue

What does this MR do?

End-to-end test to prevent https://gitlab.com/gitlab-org/gitlab-ce/issues/67295 from happening again.

Note: the new test was based on the following existing one: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/qa/qa/specs/features/browser_ui/2_plan/issue/check_mentions_for_xss_spec.rb

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content