FR: Allow restricting ad assets to subfolder

[adrienne]

I really, really want to be able to restrict the main asset upload for the ad graphic to a particular subfolder -- currently the interface for it allows selection of any image in any asset volume, and it's extremely easy for clients to put stuff in the wrong place if making a new ad via the backend.

In my opinion, a good place to put it would be in the settings for an Ad Group.