show activity stream url after submission

[lcowell]

Fixes #314

Produces output like:

Your Ruby solution for Hello World has been submitted. View it here:
http://localhost:4567/submissions/1d4b88fa148141a68ef15e522b6b980c

See related solutions and get involved here:
http://localhost:4567/tracks/ruby/exercises/hello-world

[kytrinyx]

Nice. Want to ditch the double slash on the url?

[lcowell]

It was already ditched, I just grabbed the wrong output.

edit: updated initial message with correct output.

[Tonkpils]

👍 looks great!