Customer styling

Add styling to all customer views: index, new, show, edit.

This focuses on the new _panel display, which shows a customer, the description, and a list of projects for the customer. It also has a dropdown to edit or delete a customer. There should be some items added here in the future like the priority of this customer, and the last contact with them.

This also adds some items to the customers dropdown in the navbar to show all customers or add a new one.

Closes #3