* use the AES256 cipher, be explicit that only the first line of secret_key.txt is used, and sanity check that the passphrase is long enough
* change overship of the encrypted files to the user-data user
* simplify variable names in management/backup.py
* although I appreciate long comments I am trimming the commentary about the backup migration
* revise the control panel template to not refer to the old unencrypted files
* add CHANGELOG entry