Prevent popovers from showing on group references

What does this MR do and why?

Fixes a regression where an empty popover is shown when hovering over group references

Related to #364313 (closed)

Screenshots or screen recordings

Screen_Recording_2022-06-18_at_9.28.41_AM

(@flightjs is a group so the popover should not be shown)

How to set up and validate locally

1. Add user and group references in Markdown
2. Verify that popover is only shown on user references
3. Also verify that popover works on author links

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.