Use role relative paths for references to matrix-postgres role

This makes the `sqlite://` URI match what we were using before
and what the config expects.

Some Ansible installations choke on it, it seems.
Similar to 9f00970c90

Update updating-users-passwords.md

We've hit this problem before as well. Certain Ansible installations
choke on it.

Fixed markdown error

Related to
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/740
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/686

postgres: create databases for all services

Update element-web (1.7.15 -> 1.7.16)

Update nginx (1.19.5 -> 1.19.6)

Our old (base-path -> data-path) SQLite migration can't work otherwise.

It's probably not necessary to keep it anymore, but since we still do,
at least we should take care to ensure it works.

Should have been part of 149872e00c

add option to disable nginx access log

This also adds support for installing python3-docker (not python-docker)
in systems that run Python 3.

Raspbian doesn't seem to support arm64, so this is somewhat pointless
right now.

However, they might in the future. Doing this should also unify us
some more with `setup_debian.yml` with the ultimate goal of
eliminating `setup_raspbian.yml`.

It's likely installed by default, but it doesn't hurt to specify it.
It also makes us more the same with `setup_debian.yml`.

Well, `ubuntu` or `debian`, the same key is served right now,
so it doesn't really matter.

This seems cleaner and less prone to breakage though.

Until now, we've only supported non-amd64 on Raspbian.

Seems like there are now people running Debian/Ubuntu on ARM,
so we were forcing them into amd64 Docker packages.

I've gotten a report that this change fixes support
for Ubuntu Server 20.04 on RPi 4B.

We were only defining this in `group_vars/matrix_servers`, which is
inconsistent with how we normally do things.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/570

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/641

We're getting reports of it being broken and I can confirm that the
bridge works well without it, so I don't see why we'd be asking
people to enable it.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/751

The breaking changes are properly documented in the CHANGELOG.md file.
The date used is specified as XXXX-XX-XX and should be modified at the
moment of merge.

The new variables created to the nginx reverse proxy are properly added
to the documentation.

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.

Revert "Correct inabillity for appservice-discord to connect"
This reverts commit 673e19f830.

While certain things do work even with such a local URL, sending
messages leads to an error like this:

> [DiscordBot] verbose: DiscordAPIError: Invalid Form Body
> avatar_url: Not a well formed URL.

Fixes https://github.com/Half-Shot/matrix-appservice-discord/issues/649

The sample configuration file for appservice-discord
c29cfc72f5/config/config.sample.yaml (L8)
explicitly says that we need a public URL.