Automatically enable openid listeners when ma1sd enabled

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.

CHANGELOG.md

@@ -1,3 +1,15 @@
+# 2020-12-08
+
+## openid APIs exposed by default on the federation port when federation disabled
+
+We've changed some defaults. People running with our default configuration (federation enabled), are not affected at all.
+
+If you are running an unfederated server (`matrix_synapse_federation_enabled: false`), this may be of interest to you.
+
+When federation is disabled, but ma1sd or Dimension are enabled, we'll now expose the `openid` APIs on the federation port.
+These APIs are necessary for some ma1sd features to work. If you'd like to prevent this, you can: `matrix_synapse_federation_port_openid_resource_required: false`.
+
+
 # 2020-11-27
 
 ## Recent Jitsi updates may require configuration changes