Backport some EE helper changes

What does this MR do?

This backports some EE changes made to DropdownsHelper and SelectsHelper. These changes can not easily be moved into an EE specific module, as doing so would require reimplementing all modified methods. Since there is no harm in including these changes in CE directly, we just backport them.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8604

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer