Add default on config setting to control call button in composer

This adds a default on setting that can be used to control whether call buttons
are shown in the composer via `settingDefaults` in config.json. No UI is added
to control it for now.

Fixes https://github.com/vector-im/riot-web/issues/12758

src/settings/Settings.js

@@ -517,4 +517,8 @@ export const SETTINGS = {
         displayName: _td("How fast should messages be downloaded."),
         default: 3000,
     },
+    "showCallButtonsInComposer": {
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG,
+        default: true,
+    },
 };