[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.

.env.sample

@@ -12,3 +12,12 @@ COMPOSE_FILE="compose.yml"
 
 ## Web client
 #COMPOSE_FILE="$COMPOSE_FILE:compose.mumbleweb.yml"
+
+## Server Password
+# remember to set the server-pw secret:
+#  abra app secret insert <domain> server-pw v1
+#COMPOSE_FILE="$COMPOSE_FILE:compose.serverpassword.yml"
+#SECRET_SERVER_PASSWORD_VERSION=v1
+
+## SuperUser Password
+SECRET_SUPERUSER_PASSWORD_VERSION=v1