CE backport of gitlab-ee!8936

What does this MR do?

Adds reference support for gfm_autocomplete

This allows backend to provide a reference for the dropdown entry, and it will be used as the value to be inserted when the entry is selected.

This is used for namespaces in the references.

What are the relevant issue numbers?

gitlab-ee#5606

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer