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.


Testing idea: To make it easier to the Assistant model and AssistantBase (not code modules), you can make this class store and return method arguments.

For example:

```rb
attr_accessor :last_args

def task_taskees(task_plan, taskees)
  self.last_args = []
  self.last_args[0] = task_plan
  self.last_args[1] = taskees
end
```

And then you can check the last_args in a spec.