Implement 'User sees clickable area on card'

Implement first AC in 'User responds with clickable inputs'.
Add snippet attribute to cards to store code snippet. Implement
methods on Card model to tokenize snippet.
[#51507545]
Refactor feature test setup & teardown

Refactor feature test before each and after each into modules.
Update seeders to seed positional cards. Improve templating of code
snippets.
[#51507545]
Refactor test harnesses

Improve separation between setup and init in lesson test harness.
Update Zeus to latest RC to resolve crash issues.
Implement 'User responds with clicks' US

Implement model logic to check positional answers. Implement
Backbone model for position collection and associated logic.
[Finishes #51507545]