[feat] server and superuser passwords

Impl note: server-pw can be ignored for a password-less server,
 e.g. if operator wants to run a publicly registered server:
 https://www.mumble.info/documentation/administration/config-file/#server-registration

Impl note: secret names (-pw rather than -password) are shortened due to R015.

compose.serverpassword.yml

@@ -0,0 +1,12 @@
+version: "3.8"
+
+services:
+  app:
+    secrets:
+      - source: server-pw
+        target: MUMBLE_CONFIG_SERVER_PASSWORD
+
+secrets:
+  server-pw:
+    external: true
+    name: ${STACK_NAME}_server-pw_${SECRET_SERVER_PASSWORD_VERSION}