Add placeholder links for sorting cards by vote score and by date.
Add Cards and Decks links to top bar.  Generate Decks controller.

Add flash message conditional logic to the header partial.
Add Cards and Decks links using link helpers to the top bar.
A link helper reference to Decks breaks the app since there is no
Decks controller or route.
Run `rails generate controller Decks show create update index`.
Add `resources 'decks'` to controller.