Tidy up PostModel/Collection and remove pointless Collection.js
Add form/tag models and collections

Move all App collections to App.Collections object.
Create Form/Tag models
Create Form/Tag collections
Show real categories in posts listing, and hack css for better rendering

* Grab category models in PostItemView.serializeData()
* Display categories in template
* Change categories to a list with a wrapper
* Update CSS to display categories inline