Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1393

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

Not sure if this is beneficial though.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1057

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1058

We may try to capture such calls and return a friendlier response (HTML
or JSON) saying "The Synapse Admin API is not enabled", but that may not
be desirable.

For now, we stick to what "upstream" recommends: "simply
don't proxy these APIs", which should lead to the same kind of 404 that
we have now.
See here: 6660912226/docs/reverse_proxy.md (synapse-administration-endpoints)

People who were disabling matrix-nginx-proxy (in favor of their own
nginx webserver) and also overriding `matrix_federation_public_port`,
found that the generated nginx configuration still hardcoded `8448`,
which forced their nginx server to use that, regardless of the fact
that `matrix_federation_public_port` was pointing elsewhere.

We now allow for the in-container federation port to be configurable,
and also automatically wire things properly.

We do this by creating one more layer of indirection.

First we reach some generic vhost handling matrix.DOMAIN.
A bunch of override rules are added there (capturing traffic to send to
ma1sd, etc). nginx-status and similar generic things also live there.

We then proxy to the homeserver on some other vhost (only Synapse being
available right now, but repointing this to Dendrite or other will be
possible in the future).
Then that homeserver-specific vhost does its thing to proxy to the
homeserver. It may or may not use workers, etc.

Without matrix-corporal, the flow is now:
1. matrix.DOMAIN (matrix-nginx-proxy/matrix-domain.conf)
2. matrix-nginx-proxy/matrix-synapse.conf
3. matrix-synapse

With matrix-corporal enabled, it becomes:
1. matrix.DOMAIN (matrix-nginx-proxy/matrix-domain.conf)
2. matrix-corporal
3. matrix-nginx-proxy/matrix-synapse.conf
4. matrix-synapse

(matrix-corporal gets injected at step 2).

Not sure if it breaks with them or not, but no other directive
uses quotes and the nginx docs show examples without quotes,
so we're being consistent with all of that.

Code was clean up and simplified to make it simpler and easier to
maintain. No features were modified.

The different configurations are now all lower case, for consistent
naming.

`matrix_nginx_proxy_ssl_config` is now called
`matrix_nginx_proxy_ssl_preset`. The different options for "modern",
"intermediate" and "old" are stored in the main.yml file, instead of
being hardcoded in the configuration files. This will improve the
maintainability of the code.

The "custom" preset was removed. Now if one of the variables is set, it
will use it instead of the preset. This will allow to mix and match more
easily, for example using all the intermediate options but only
supporting TLSv1.2. This will also provide better backward
compatibility.

A new variable called `matrix_nginx_proxy_ssl_config` is created for
configuring how the nginx proxy configures SSL. Also a new configuration
validation option and other auxiliary variables are created.

A new variable configuration called `matrix_nginx_proxy_ssl_config` is
created. This allow to set the SSL configuration easily using the
default options proposed by Mozilla. The default configuration is set to
"Intermediate", removing the weak ciphers used in the old
configurations.

The new variable can also be set to "Custom" for a more granular control.
This allows to set another three variables called:

- `matrix_nginx_proxy_ssl_protocols`,
- `matrix_nginx_proxy_ssl_prefer_server_ciphers`
- `matrix_nginx_proxy_ssl_ciphers`

Also a new task is added to validate the SSL configuration variable.

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

add doc

This makes all containers (except mautrix-telegram and
mautrix-whatsapp), start as a non-root user.

We do this, because we don't trust some of the images.
In any case, we'd rather not trust ALL images and avoid giving
`root` access at all. We can't be sure they would drop privileges
or what they might do before they do it.

Because Postfix doesn't support running as non-root,
it had to be replaced by an Exim mail server.

The matrix-nginx-proxy nginx container image is patched up
(by replacing its main configuration) so that it can work as non-root.
It seems like there's no other good image that we can use and that is up-to-date
(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).

Likewise for riot-web (https://hub.docker.com/r/bubuntux/riot-web/),
we patch it up ourselves when starting (replacing the main nginx
configuration).
Ideally, it would be fixed upstream so we can simplify.

Fixes #83 (Github issue)