fix: add base_topic to config.json schema (#623)

Make `mqtt.base_topic` editable from the addon page

Author: JSteunou

zigbee2mqtt/config.json

@@ -60,7 +60,8 @@
       "key": "str?",
       "cert": "str?",
       "user": "str?",
-      "password": "str?"
+      "password": "str?",
+      "base_topic": "str?"
     },
     "serial": {
       "port": "str?",