Camera roll photos can be run through predictions model for complete offline experience

[albullington]

Now that I've added functionality to let users save observations (and earn badges) from the AR camera 100% offline, it would be nice to take the camera roll experience offline as well.

• Camera should accept a URI from the camera roll and run the corresponding image through the classifier
• Camera should return a prediction for that image (in the same format as other predictions)

Not exactly sure what implementation looks like here since the camera is unmounted while the user is on the photo gallery tab, but that's the general idea. Happy to sync if needed.