Add GOLD/SILVER ratio indicator to dashboard

Add gold_silver_ratio indicator (xyz:GOLD / xyz:SILVER) with fallback
reference of 61.59, mirroring the existing WTI/BRENT ratio setup.
Also register xyz:GOLD and xyz:SILVER in WATCHED_COINS and data_fetcher
defaults so the candle data is fetched for the new indicator.

_data/indicators.json

@@ -8,5 +8,15 @@
         "show_deviation": true,
         "min_data_points": 100,
         "fallback_reference": 0.96065
+    },
+    "gold_silver_ratio": {
+        "display_name": "GOLD/SILVER",
+        "type": "ratio",
+        "numerator": "xyz:GOLD",
+        "denominator": "xyz:SILVER",
+        "changes": ["1h", "1d"],
+        "show_deviation": true,
+        "min_data_points": 100,
+        "fallback_reference": 61.59
     }
 }