Add link to issue on epic

What does this MR do?

Added a link in the system note for an issue that was added to an epic.

What are the relevant issue numbers?

Closes #6318 (closed) - System notes generated on epics should link to referenced issues.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?