Enable "Source URL" submission on puzzles not in the current mendicant exam

[jordanbyron]

We will probably need to add another attribute on Puzzle. Puzzle#allow_source_url or Puzzle#share_solutions, something along those lines.

Most of the code is in place but commented out:

https://github.com/rmu/puzzlenode/blob/master/app/views/users/submissions/_source_url.html.haml

[justinramel]

I'd like to pick this one up.

I've had a look and think I need to:

• add migration to add allow_source_url attribute to puzzles
• add allow_source_url field to puzzle admin page
• show source_url partial when solution is correct and allow_source_url is set

[jordanbyron]

@justinramel that sounds about right. Greg and I aren't exactly sure how well this is going to work, but I'd like to test out this method and see if people start using it.

Thanks!