Implement Rental Interval

[christroutner]

Add a field to the device public model of rentalInterval with potential values of hourly, daily, weekly, monthly, and yearly. This will eventually get a UI component in the Dashboard. The user will set the rental interval for the device when they add a new device to the Owned Devices view.

[christroutner]

Don't forget to add unit tests for this in a03-device.spec.js