schemas: Add missing display_default properties for Auspice config v2

Resolves <#911> which noted
"language".  In confirming that, I cross-referenced Auspice's view
settings doc¹ and code² which revealed "sidebar" and "panels" too.
Clarifies the purpose of the two different "panels" keys.

¹ https://docs.nextstrain.org/projects/auspice/en/stable/advanced-functionality/view-settings.html#dataset-json-configurable-defaults
² https://github.com/nextstrain/auspice/blob/ce8380f7/src/actions/recomputeReduxState.js#L699-L718

augur/data/schema-auspice-config-v2.json

@@ -181,6 +181,25 @@
                 },
                 "transmission_lines": {
                     "type": "boolean"
+                },
+                "language": {
+                    "type": "string",
+                    "minLength": 1,
+                    "description": "A BCP 47 language tag specifying the default language in which to display Auspice's interface (if supported)"
+                },
+                "sidebar": {
+                    "type": "string",
+                    "enum": ["open", "closed"]
+                },
+                "panels": {
+                    "type": "array",
+                    "description": "The panels to display by default.",
+                    "$comment": "optional, but if present must not be empty",
+                    "minItems": 1,
+                    "items": {
+                        "type": "string",
+                        "enum": ["tree", "map", "frequencies", "entropy"]
+                    }
                 }
             }
         },
@@ -197,7 +216,7 @@
         },
         "panels": {
             "type": "array",
-            "description": "Which panels should auspice display",
+            "description": "The panels available for display.",
             "$comment": "optional",
             "minItems": 1,
             "items": {