* 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
[Josh merged some subsequent commits:]
* Guard via idempotency against termination between migration operations
* Final corrections and tweaks
* Pass passphrase through to all duplicity calls
Empirical evidence (a failed cron job) shows that cleanup requires the
passphrase (so it presumably needs to decrypt metadata), and though
remove-older-than has been working fine without it, it won't do any harm
to set it in case that changes or there are any special cases.
* Add back the archive-dir override but locate it at STORAGE_ROOT/backup/cache
