Clean up setting definition to follow surrounding practices

It's not perfect, but we're at least okay with it.

src/settings/Settings.js

@@ -152,9 +152,10 @@ export const SETTINGS = {
         default: null,
     },
     "feature_cross_signing": {
-        //isFeature: true,
-        displayName: _td("Enable cross-signing to verify per-user instead of per-session (in development)"),
-        supportedLevels: LEVELS_FEATURE,
+        // XXX: We shouldn't be using the feature prefix for non-feature settings. There is an exception
+        // for this case though as we're converting a feature to a setting for a temporary safety net.
+        displayName: _td("Enable cross-signing to verify per-user instead of per-session"),
+        supportedLevels: ['device', 'config'], // we shouldn't use LEVELS_FEATURE for non-features, so copy it here.
         default: true,
     },
     "feature_event_indexing": {