The old variables still work. The global lets us avoid
auto-detection logic like we're currently doing for
`matrix_nginx_proxy_proxy_matrix_federation_api_enabled`.

In the future, we'd just be able to reference
`matrix_homeserver_federation_enabled` and know the up-to-date value
regardless of homeserver.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2806

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.

This avoids having to create a new room and to click the "Add widgets, bridges & bots" link (Formerly the four-squares-icon)

ma1sd requires the openid endpoints for certain functionality.
Example: 90b2b5301c/src/main/java/io/kamax/mxisd/auth/AccountManager.java (L67-L99)

If federation is disabled, we still need to expose these openid APIs on the
federation port.

Previously, we were doing similar magic for Dimension.
As per its documentation, when running unfederated, one is to enable
the openid listener as well. As per their recommendation, people
are advised to do enable it on the Client-Server API port
and use the `federationUrl` variable to override where the federation
port is (making federation requests go to the Client-Server API).

Because ma1sd always uses the federation port (unless you do some
DNS overwriting magic using its configuration -- which we'd rather not
do), it's better if we just default to putting the `openid` listener
where it belongs - on the federation port.

With this commit, we retain the "automatically enable openid APIs" thing
we've been doing for Dimension, but move it to the federation port instead.
We also now do the same thing when ma1sd is enabled.

Improved formatting for a header that was attached to the previous paragraph (...by inserting a newline).

Potentially the CURLing could be a better approach, since then you don't leave spare sessions hanging.

Fixes #586 (Github Issue)

This can be removed once turt2live/matrix-dimension#345 is resolved.

I am new to synapse. Thanks so much for these playbooks!

I wasn't sure how you actual activate dimensions after setting these variables. Should you re run 

ansible-playbook -i inventory/hosts setup.yml --tags=setup-all
ansible-playbook -i inventory/hosts setup.yml --tags=start

? If so perhaps you could tell the readers to do this somewhere in the help file

Thanks =]