Display link to each review note in text email

CE port at here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26695

What does this MR do?

Currently the HTML email would display a link to the note, whereas the text email would not. This updates the text email's behavior to be in sync with HTML email.

This is HTML email as reference, note the "on test" link:

This is the updated text email, with link appended:

What are the relevant issue numbers?

Closes #10220 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• 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
• 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?