Web OTA: check authentication result before accepting payload

[mcspr]

Using HTTP upgrade method described in the wiki and omitting auth headers:
https://github.com/xoseperez/espurna/wiki/OTA

curl -XPOST  \
  -H "Content-Type: multipart/form-data" \
  -F "filename=@.pioenvs/esp8266-1m-ota/firmware.bin" \
  http://192.168.4.1/upgrade

Causes file handler to process OTA file and write it to flash and only then to go into the request handler, where it will issue auth error.

[mcspr]

TODO: use AsyncWebHandler::setAuthentication(username, password) instead of doing manual auth?
Per dff81d6, /ws is kept with ticker-based auth. Every other handler return value of _server->on(...)) needs to have setAuthentication.
Loosing log message and an easy ability to refresh the password. Password can be updated via change detection in the reload method and applied to local handlers ref storage. Or, handlers can be accessed by subclassing server class and accessing _handlers attr