Allow user to specify which application to book under

[DustinMorado]

If a user has two applications assigned to their user account only the most recent one is used for booking.

Can be tested by creating a new application with 0 days for any bookable resource that requires an application and assigning it to a user account already with an application that still has remaining days bookable, and trying to book a session.

Also if a user is has two applications assigned it should be possible for them to choose which application to book as so that the proper invoicing can be done.