Support key rotation

[kumar303]

Unless it's not documented, there is no way to rotate AES keys in production while maintaining historic data integrity. We would probably need something like manage.py rotate_aes_keys --key_name=... --old-key-file=... and that could loop through and update all fields with the new key. This might cause intermittent errors while the update is in progress so perhaps there is a better way to do it.