Combine cross signing and verification over DM feature flags

This means we can just make the new member info panel support
cross-signing exclusively rather than having to try & make it
temporarily support both

src/components/structures/MatrixChat.js

@@ -1466,7 +1466,7 @@ export default createReactClass({
             }
         });
 
-        if (SettingsStore.isFeatureEnabled("feature_dm_verification")) {
+        if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
             cli.on("crypto.verification.request", request => {
                 let requestObserver;
                 if (request.event.getRoomId()) {