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.


It should work, but I think it only makes sense to do it if all or almost all types of task_steps have the url and content attributes. Regardless, the `property :content` and `property :url` could just be moved into the TaskStepProperties (or another similar module, for the TaskSteps that have resources)