Only enable Field or View option for data source

When adding or editing a subpage, the admin can choose to use a Field or View as the data source for that subpage. Right now, both of these dropdowns are available. The admin should only be able to make a selection in one of the two.

Add radio buttons to select select which dropdown should be visible. This blog post details how to do this with #states. This will need to be ported to 6.x-1.x as well.