Add ability to shout

* Add Shout model
* Add shout form to dashboards#show
* Add shouts#create
* Extract shout partial
* Add has_many :shouts to User
User can follow each other

* Create FollowingRelationship between users
* Add a button to Follow/Unfollow a user
* Don't display the button on the current user's page
* Ensure that users can only follow each other