Show the mentor if the student is in the mentored track

I like to encourage my non-mentored students to join the mentored track, but sometimes it is hard to see whether they are already members.

No implementation files or tests are named. Start by checking the current contribution location in exercism/exercism, since this idea is in the paused exercism/ideas repository; define where mentor-facing membership information should appear and confirm that mentors can distinguish mentored-track students when the work is complete.

Written by the indexing model from the issue text.