Make change to install roundcube from wheezy backports on Debian 7 work with Ubuntu 14.04

- 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.

Resolves #266.

/etc/apache2/sites-available and old (dangling) symlinks in
/etc/apaches/sites-enabled.
Also, remove unnecessary apache2 restart after creating a new
virtualhost but not yet enabling it.

I guess the .conf appendix is a change introduced in apache 2.4.
This change should make it backward compatible with apache 2.2.

On Ubuntu 14.04 LTS, a2ensite automatically appends ".conf" to the filename it looks for in /etc/apache2/sites-available/
Therefore, the file "/etc/apache2/sites-available/roundcube" must be renamed to
"/etc/apache2/sites-available/roundcube.conf".

Security issue:
This file must be owned by root, otherwise it is a huge security issue (User www-data could modify the file and get root at next restart of apache).

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.

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).

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>

Refs #39