- Added backports to apt configuration.
- Updated roundcube task to use the roundcube backport.
- Updated roundcube's main.inc.php configuration file to use the one included
in the backport distribution.  Migrated configuration parameters from the
existing configuration file except in cases where it looked like the Roundcube
team was changing defaults that were not touched by Sovereign's configuration.
I may have gotten some wrong; needs review by Sovereign maintainer.

The previous behaviour relied on managesieve copying over the
.dovecot.sieve file into the user's directory. I found this to be
particularly fragile. For instance, re-deploying roundcube without
dovecot could overwrite the .dovecot.sieve symlink and break
managesieve.

A better approach is to use the global sieve configuration that
roundcube provides and not mess with dovecot's files directly.

These are the changes necessary to enable Roundcube's manage sieve plugin to mange server-side sieve filters.

Signed-off-by: PajamaSoft <support@pajamasoft.com>