- forego removing Docker images - it's not effective anyway, because it
  only removes the last version.. which is a drop in the bucket, usually

- do not reload systemd - it's none of our business. `--tags=start`,
  etc., handle this

- combine all uninstall tasks under a single block, which only runs if
  we detect traces (a leftover systemd .service file) of the component.
  If no such .service is detected, we skip them all. This may lead to
  incorect cleanup in rare cases, but is good enough for the most part.

We had checks to avoid stopping/deleting systemd services for workers
that used to exist and will continue to exist, but we were deleting
config files for workers each time.. Only to recreate them again later.

This lead to:

- too many misleading "changed" tasks
- too much unnecessary work
- potential failures during playbook execution possibly leaving the
  system in a bad state (no worker config files)

This was useful when the order of these roles in relation to Synapse
mattered (when we were injecting stuff into Synapse variables during
runtime). This is no longer the case since 0ea7cb5d1812a88, so all of
this can be removed.

These `init.yml` (now `inject_into_nginx_proxy.yml`) tasks do not need
to `always` run. They only need to run for `setup-all` and
`setup-nginx-proxy`. Unless we're dealing with these 2 tags, we can
spare ourselves a lot of work.

This patch also moves the `when` statement from `init.yml` into
`main.yml` in an effort to further optimize things by potentially
avoiding the extra file include.

This caused push notifications to be delivered more than once
if multiple `pusher` workers were enabled.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2228

This fixes a regression since the change done in c1c152f7ac.
When another role (say `matrix-jitsi`) included `roles/custom/matrix-base/tasks/util/ensure_openssl_installed.yml`,
which then included `{{ role_path }}/tasks/util/ensure_openssl_installed_DISTRO.yml`,
that `role_path` variable would end up being the parent role
(`matrix-jitsi`) and not the `matrix-base` role, so we'd get a failure.

An alternative solution may have been to avoid using `role_path`, but
importing roles properly (like we've done in this patch) sounds like a better way.

Unfortunately, `import_role` fails if `tasks_from` is something like
`util/ensure_openssl_installed` (containing a `/`), so I had to move
these utils out of `util/`.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2228

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.