Added Navbar to all pages in the `card_inventory` app.

[ashleawalker29]

• Changed all instances of " to ' for consistency.
• Added generic navbar that can be personalized per page using the navbar_title and navbar_dropdown blocks.
• Generalized the sets_index page so that all card set links are contained within the dropdown menu.
• Added title response within the sets_index model for use by the navbar_title block within the HTML file.

Closes #15

Verified that the webpage renders as desired. The navbar dropdown menu contains all the links that were previously buttons. The navbar title is consistent with the render of views.sets_index.