Refactor User model for clarity
Make shouts polymorphic

* Create TextShout model
* Update shouts table and move current shouts to text_shouts table
* Rename shout.content to shout.body
* Define polymorphic relationship in Shout model
* Update views and controllers to display txt shouts from updted models
Add image shouts

* Use paperclip gem for file uploads
* Ignore /public/system in git
* Create ImageShout model and migration
* Add image upload form to dashboards#show
* Add route and controller for image_shouts#create
* Refactor some views and partials to take advantage of polymorphism