Add user popovers to system notes

What does this MR do?

1. Show user popovers for system notes
2. Make GFM use the .js-user-link selector instead of gfm-project_member (which I think is more for CSS's use)
3. ???

Screenshots

Before	After
Screen_Recording_2019-12-06_at_15.05.05

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides