Move complement setup stuff into the Synapse repo (#12404)

Fixes matrix-org/complement#330 (or it will, once we remove the old files).

It's not quite a lift-and-shift: I've also taken the opportunity to get rid of the custom CA that we used to use to sign the TLS certs, which has been superceded by the CA exposed by Complement.

docker/complement/conf/start.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+set -e
+
+sed -i "s/SERVER_NAME/${SERVER_NAME}/g" /conf/homeserver.yaml
+
+# Add the application service registration files to the homeserver.yaml config
+for filename in /complement/appservice/*.yaml; do
+  [ -f "$filename" ] || break
+
+  as_id=$(basename "$filename" .yaml)
+
+  # Insert the path to the registration file and the AS_REGISTRATION_FILES marker after 
+  # so we can add the next application service in the next iteration of this for loop
+  sed -i "s/AS_REGISTRATION_FILES/  - \/complement\/appservice\/${as_id}.yaml\nAS_REGISTRATION_FILES/g" /conf/homeserver.yaml
+done
+# Remove the AS_REGISTRATION_FILES entry
+sed -i "s/AS_REGISTRATION_FILES//g" /conf/homeserver.yaml
+
+# generate an ssl key and cert for the server, signed by the complement CA
+openssl genrsa -out /conf/server.tls.key 2048
+
+openssl req -new -key /conf/server.tls.key -out /conf/server.tls.csr \
+  -subj "/CN=${SERVER_NAME}"
+openssl x509 -req -in /conf/server.tls.csr \
+  -CA /complement/ca/ca.crt -CAkey /complement/ca/ca.key -set_serial 1 \
+  -out /conf/server.tls.crt
+
+exec python -m synapse.app.homeserver -c /conf/homeserver.yaml "$@"
+