Added firmware update to Web UI. Firmware can be updated by uploading file using browser or curl.

I first tried to use multipart/form form but it turns out it is difficult to handle multipart/form data.

I searched a bit and I found that javascript can be used to upload file without any encoding/encapsulation. So ESP32 simply writes recieved body to flash. This is same behavior as previous version, but I added callback functions to receive start/end of update and display start/end status.