Cannot mark large feeds as read

[jtojnar]

I have a feed with over a thousand items but when I scroll down and press the “Mark as read” button, it fails with the following error:

{"status":"Internal Server Error","code":500,"text":"Fatal error: Unknown: Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini.","level":0}

We should probably send the request as application/json instead of application/x-www-form-urlencoded.