Make topic mandatory

This pull request makes an associated topic required on all editions that can have topics. If you can't find a topic to use there's a box to enter a suggestion for a new topic, which is sufficient for passing validation.

![topic-suggestion](https://f.cloud.github.com/assets/74812/1549820/d0ae4a44-4df1-11e3-9314-bc185fd59925.png)

In addition to implementing the story, I've also cleaned up the topics view in admin and remove the cached counts and scopes from topics since they all have policies and documents now.

https://www.pivotaltracker.com/story/show/59495626