MAR-227: Drop the workaround for older Postfix TLS protocol configuration syntax:

- Since the roles have dropped support for older versions of
Debian/Postfix, no need to use the backwards compatible syntax any
longer.

MAR-195: Reject mails with non-matching sender for logged-in users:

- Prevents logged-in user from setting "MAIL FROM" with e-mail address
that does not match either his own account or one of the aliases
that the user receives mails from.
- Reworked the submission port configuration to restrict only based on
sender-related restrictions, merging some of the newer changs from
the workaround.org instructions.

MAR-196: Simplify allowed TLS protocol configuration for mail_server role:

- Accept minimum version allowed instead of arbitrary list.
- Fixes deprecation warnings in Dovecot logs (ssl_protocols ->
ssl_min_protocol transition).

MAR-189: Merge some configuration changes from Debian Bullseye for mail server configuration files:

- Get rid of using the session cache databases for TLS for smtpd -
this is no longer the preferred way to handle it (RFC 5077 TLS
session tickets should be used).
- Indicate that the delivery through external commands is final (X
flag of the pipe(8) mechanism). Affects the delivery status
notifications, changing them from "relayed" to "delivered".

MAR-175: Update Postfix configuration, merging changes from Debian Buster stock installation and latest ISPmail tutorials:

- Added commented-out section for TLS parameters from Debian Buster
default configuration. This should slightly help with handling the
diff's in the future.
- Added a number of "missing" configuration parameters from the Debian
Buster stock configuration.
- Separate relay-related restrictions from spam-related restrictions.
- Updated configuration for RBLs.
- Updated default service definitions in the master.cf configuration
to be in-line with options defined in Debian Buster. This is
primarily revolving around the change to the chroot
option. Previously the default was to chroot the processes, while
new default is not to do so. This required marking a number of
services to be explicitly chroot'ed.
- Updated the submission service definition in master.cf configuration
file. Some minor changes were made, like switching to using the
smtpd_relay_restrictions instead of
smtpd_recipient_restrictions (better suited configuration option for
this use-case since it's not spam-related), being more explicit
around TLS being required for authentication.

MAR-28: Impelmented general and SMTP-related tests for mail_server:

- Increased amount of memory in test instances to 768MB.
- Fixed playbook for runs against client test instances.
- Fixed typos and invalid settings in test playbook.
- Switched to expanded syntax in tasks used for deploying TLS keys and
certificates in order to avoid mangling of TAB characters.
- Fixed missing permissions set-up for Postfix main configuration file.
- Fixed deployment of Postfix master.cf configuration file to take into account
configurable virtual mail user.
- Implemented tests covering SMTP functionality.