don't spawn an extra container
run pg_dumpall within matrix-postgres instead, ensures correct version
store under /matrix so a backup of the folder will contain a DB dump
use absolute paths just in case something in the ENV is messed up

.. and other documentation improvements.

We mainly switch the anonymous metrics viewing variable
to false, along with other wording changes.

Added note about cloudflare, related to #821

Warn users of the known bug https://github.com/turt2live/matrix-dimension/issues/395

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/682

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/795
and https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/822

A better fix will come later: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/825

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.

This fixes things to make them compatible
with the changes done in 95346f3117

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.

Prompted by this: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/779#issuecomment-764807507

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/756

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/737

I feel like timers are somewhat more complicated and dirty (compared to
cronjobs), but they come with these benefits:

- log output goes to journald
- on newer systemd distros, you can see when the timer fired, when it
will fire, etc.
- we don't need to rely on cron (reducing our dependencies to just
systemd + Docker)

Cronjobs work well, but it's one more dependency that needs to be
installed. We were even asking people to install it manually
(in `docs/prerequisites.md`), which could have gone unnoticed.

Once in a while someone says "my SSL certificates didn't renew"
and it's likely because they forgot to install a cron daemon.

Switching to systemd timers means that installation is simpler
and more unified.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/786

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/778

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.