Use built in duplicity encryption (GPG) for backups, closes #362, closes #363

[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

management/templates/system-backup.html

@@ -54,7 +54,7 @@ function show_system_backup() {
     "GET",
     { },
     function(r) {
-      $('#backup-location').text(r.encdirectory);
+      $('#backup-location').text(r.directory);
       $('#backup-encpassword-file').text(r.encpwfile);
 
       $('#backup-status tbody').html("");
@@ -72,7 +72,7 @@ function show_system_backup() {
         tr.append( $('<td/>').text(b.date_str + " " + r.tz) );
         tr.append( $('<td/>').text(b.date_delta + " ago") );
         tr.append( $('<td/>').text(b.full ? "full" : "increment") );
-        tr.append( $('<td style="text-align: right"/>').text( nice_size(b.encsize)) );
+        tr.append( $('<td style="text-align: right"/>').text( nice_size(b.size)) );
         if (b.deleted_in)
           tr.append( $('<td/>').text(b.deleted_in) );
         else
@@ -80,7 +80,6 @@ function show_system_backup() {
         $('#backup-status tbody').append(tr);
 
         total_disk_size += b.size;
-        total_disk_size += b.encsize;
       }
 
       $('#backup-total-size').text(nice_size(total_disk_size));