Queue classifications

Closes #48 
The `#first_or_create` call should only create the preferences the first time it's called. We could make the the `#should_create_project_preference?` method check existence instead if you felt that would be clearer. 

```ruby
def should_create_project_preference?
  !classification.anonymous? && !UserProjectPreference.exists?(user: user, project: project)
end

def create_project_preference
  UserProjectPreference.create!(user: user, project: project...etc)
end
```
I'm not sure one is better, and I don't think there's a way to avoid having to check for the preference model existence on every classification.