Username is a text field on sessions#new
Add Home page and Dashboard page

* Require login in application controller (for all actions)
* Add root route and dashboard route
* Add homes#show and dashboards#show
* Extract partials for sign in and sign up
* Extract partial for form errors
* Add nav links to header in application layout
Minor view tweaks

* Display username when signed in
* Add headings to sign up and sign in forms
Add Shouts model

* Create and run migration for Shouts
* Add has_many reference in User model
Implement shouting and viewing shouts

* Add form to dashboards#show for creating a shout
* Add route for shouts#create
* Add shouts controller and implement shotus#create
* Display the current user's shouts on dashboards#show, using a partial
Sort shouts in reverse-chronologically