Disallow multiple form submissions

[therealklanni]

Store a has_applied Boolean or applied_at Date in Mongo to check whether user has already submitted an application.

If User has already applied, the solution can be one of the following:

• Show an error if they have already applied
• Display a message on the form ("Your application has been submitted" ?) and disable form
• Show an alternate view (date they submitted application and/or other details?). An alternate view may allow us to support other features.