fix: Show outdated browser warning immediately

The outdated browser warning logic has been updated to ensure that it
gets shown immediately fot new installations if applicable.

We have a timer setup to ensure we don't bother users too often with
this warning; it recurs every two days. However, it was initialized
to the current date, so new users would not see it for two days.

app/scripts/controllers/app-state.js

@@ -43,7 +43,7 @@ export default class AppStateController extends EventEmitter {
       fullScreenGasPollTokens: [],
       recoveryPhraseReminderHasBeenShown: false,
       recoveryPhraseReminderLastShown: new Date().getTime(),
-      outdatedBrowserWarningLastShown: new Date().getTime(),
+      outdatedBrowserWarningLastShown: null,
       nftsDetectionNoticeDismissed: false,
       showTestnetMessageInDropdown: true,
       showBetaHeader: isBeta(),