Link back to track after submitting in mentored mode

Once you've submitted a solution, you should be encouraged to go solve other exercises that are available to you. E.g. _A mentor will give you some feedback. Explore other exercises you can do at SITE_URL/my/tracks/:track_

We will need the API to pass back whether something is in mentored mode when writing the solution file, or as a response to submitting. See also https://github.com/exercism/cli/issues/672