MAR-192: Update compatibility-related entries for Postfix main configuration file:

- Default value for append_dot_mydomain has been "no" for a while now,
no need to set it explicitly.
- Prefer the whitelist/blacklist instead of allowlist/denylist
keywords in logs.

MAR-192: Reformat the Postfix main configuration file for mail_server:

- Improve the comments and split up the config with section titles.
- Drop the deprecated/unnecessary option smtpd_use_tls (already
covered with smtpd_tls_security_level).

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-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: Mail server should be opportunistic in using TLS when delivering mail to remove servers:

- Previously the mail server would only deliver mails over plaintext.
- Deploy a simple SMTP server on both client1/client2
machines. Servers are set-up to require/refuse the STARTTLS over
SMTP.
- Added tests for checking if STARTTLS is used when available for mail
delivery.
- Fixed the wrong configurtion (making sure the TLS security level is
properly set for Postfix).

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-27: Added initial scaffolding for testing mail_forwarder role:

- Fixed issues reported by Ansible linting check (some mode-related syntax and
one ignore.
- Added Molecule configuration for testing mandatory and optional
parameters. Covers both Debian Jessie and Debian Stretch.
- Added test playbook for setting-up the test instances. A helper relay mail
server.
- Updated both mail_server and mail_forwarder to fall-back to using
native (/etc/hosts) resolving if DNS fails. Solves issue with test environment
not having proper DNS set-up for all domains etc.
- Added a number of data/config files associated with tests.
- Added dummy test file.