add CORS headers

this will allow external portal on the network to send credentials via browser.

WiFiManager.cpp

@@ -1159,6 +1159,7 @@ void WiFiManager::handleWifiSave() {
   page += FPSTR(HTTP_END);
 
   server->sendHeader(FPSTR(HTTP_HEAD_CL), String(page.length()));
+  server->sendHeader(HTTP_HEAD_CORS, HTTP_HEAD_CORS_ALLOW_ALL);
   server->send(200, FPSTR(HTTP_HEAD_CT), page);
 
   DEBUG_WM(DEBUG_DEV,F("Sent wifi save page"));