Add close_select2 method to select2_helper

What does this MR do?

CE Port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12096

This MR adds a select2 helper method.

Does this MR meet the acceptance criteria?

Conformity

• 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