WIP Assistant / TaskPlans / Sprint 3

Don't merge, WIP.

To test it out, switch to this branch and then run:

```
rake db:drop
rake db:migrate
rake sprint:003
rails server
```

Go to `http://localhost:3001`, sign in as the sole user (printed out in the sprint:003 run above.


Did this:

    def task_taskees(task_plan, taskees)
      raise AbstractMethodCalled
    end

this error is defined in OpenStax Utilities.