Issues and Pull Requests were not migrated to the new
organization/repository, so `matrix-org/synapse/pull` and
`matrix-org/synapse/issues` references were kept as-is.

`matrix-org/synapse-s3-storage-provider` references were also kept,
as that module still continues living under the `matrix-org` organization.

This patch mainly aims to change documentation-related things, not actual
usage in full yet. For polish that, another more comprehensive patch is coming later.

This also adds labels for Synapse. Support for other homeservers and
components will be added later.

This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

Recently, documentation on Synapse has been changed from .rst to .md. Therefore, the current links for the purge history API were resulting in a 404 error.

This reverts commit 1fb54a37cb.

Seems like it's been pulled or something. It used to exist, but not
anymore. Not sure what's going on.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1017

Related to
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

This reverts commit f825c7c263.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1010

Also removes some duplicate table of contents entries in maintenance-synapse.md

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/746

Fixes #586 (Github Issue)

Fixes #465 (Github Issue).

Related to https://github.com/xwiki-labs/synapse_scripts/pull/12.

We do restart Synapse explicitly, but some other services
(bridges, matrix-corporal, ..) may not restart sometimes.

It's best to restart all services explicitly.