Prevent deleting 'uncategorized' category

Hi!

I'm looking for some feedback on this. This prevents deleting the 'uncategorized' category referenced in [this bug report on meta discourse](http://meta.discourse.org/t/uncategorized-pinned-post-not-shown-for-anon-user/10717).

This solves the issue by hiding the Delete Category Button, but I'm interested to see:
1. if there is a better way than 'cat_id !== Discourse.Site.currentProp("uncategorized_category_id"))'. The `edit_category_controller` references `isUncategorized`, but I couldn't figure out how to use that (am not very familiar with ember or js).
2. Would it be a good idea to add a `before_destroy` filter to the Category model prevent deleting the 'uncategorized' category?

Looking for any other feedback as well!
Thanks