Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2211

Signed-off-by: Sebastian Gumprich <github@gumpri.ch>

This allows people to augment the Synapse image with custom tools and
addons without having to rebuild it from scratch.

If customizations are enabled, the playbook will build a new
`localhost/matrixdotorg/synapse:VERSION-customized` image
on top of the default one (`FROM matrixdotorg/synapse:VERSION`)
and with custom Dockerfile build steps.

For servers that self-build the Synapse image, the Synapse image will be
built first, before proceding to extend it the same way.

In the future, we'll also have easy to enable Dockerfile build steps
for modules that the playbook supports.

Fixes a regression introduced in d1b5681ba0.

Looks like `.finished` is a property on the main object, not on some
`.async_result` object.

We were previously trying to reach into `.rc`,
but there's no such thing if `async_result.finished == 0`.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2153

This mostly fixes `key-order` warnings around
`block` statements.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1982

One that doesn't trip up ansible-lint, causing `load-failure` errors.

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#var-spacing

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#no-changed-when

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1749

Official support ends today (2021-11-11).
Synapse still supports v9.6, but we'd better force users to transition
to newer versions anyway.

Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

It's mildly annoying when trying to execute these scripts while logged
in as a regular user, as the missing execute permissions will hinder
autocompletion even when trying to use with sudo.

These shell scripts don't contain secrets, but may fail when ran by a
regular user. The failure is due to the lack of access to the /matrix
directory, and does not result in any damage.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Well, they still do go to that DB by default,
but our docs give a better command to users, which would do the right
thing.

In short, this makes Synapse a 2nd class citizen,
preparing for a future where it's just one-of-many homeserver software
options.

We also no longer have a default Postgres superuser password,
which improves security.

The changelog explains more as to why this was done
and how to proceed from here.

We'll be adding `.timer` units later on, so it's good to be
more explicit.

We need to suppress systemd service-stopping requests in certain rare
cases like https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/771

That issue seems to describe a case, where a migration from mxisd to
ma1sd was happening (DB files had just been moved), and then we were
attemping to stop `matrix-ma1sd.service` so we could import that database into
Postgres. However, there's neither `matrix-mxisd.service`, nor
`matrix-ma1sd.service` after `migrate_mxisd.yml` had just run, so
stopping `matrix-ma1sd.service` was failing.

In cases where pgloader is not enough and we need to do some additional
migration work after it, we can now use
`additional_psql_statements_list` and
`additional_psql_statements_db_name`.

This is to be used when migrating `matrix-registration`'s data at the
very least.

We were running into conflicts, because having initialized
the roles (users) and databases, trying to import leads to
errors (role XXX already exists, etc.).

We were previously ignoring the Synapse database (`homeserver`)
when upgrading/importing, because that one gets created by default
whenever the container starts.

For our additional databases, it's a similar situation now.
It's not created by default as soon as Postgres starts with an empty
database, but rather we create it as part of running the playbook.

So we either need to skip those role/database creation statements
while upgrading/importing, or to avoid creating the additional database
and rely on the import for that. I've gone for the former, because
it's already similar to what we were doing and it's simpler
(it lets `setup_postgres.yml` be the same in all scenarios).