Several fixes/improvements

* Added CORS support via the django-cors-headers package that allows whitelisting of specific sites (and whitelisted the appropriate servers). Necessary because Neurosynth needs to pull binary images from Neurovault for decoding. Eventually I think we should allow all sites to make CORS requests, but for now let's make sure this works.

* Added confirm dialogs when user clicks a deletion option, and made the buttons red and moved them to the right.

* JSON conversion now only occurs if the Image's FileField is dirty.