Link to community solutions after submitting when in Independent Mode

[kytrinyx]

We should print the link for people to browse the community solutions if they're not in mentored mode.

At the bottom of the submit command, we can add something like this:

 	fmt.Fprintf(Out, "    %s\n\n", solution.URL)
+
+	siteURL := config.InferSiteURL(usrCfg.GetString("apibaseurl"))
+	msg = `
+    View other solutions to this exercise at:
+    %s/tracks/%s/exercises/%s/solutions
+
+`
+	fmt.Fprintf(Err, msg, siteURL, solution.Track, solution.Exercise)

However, we will need to get the API to tell us which mode a solution is being solved in so that we can check for that before printing it, either as part of the solution metadata when downloading, or as part of the response when submitting. See also #673