Use of absolute paths causes failures with subpath-based reverse proxying

[gongloo]

It'd be great to see relative paths used throughout the JS code.

In use cases involving reverse proxies, it's common to host multiple devices behind a single HTTP proxy using subpath-based routing of requests. In my case, I've got a thermostat device accessible at http://hostname/thermostat. I can of course hit http://hostname/thermostat/update, but the OTA start requests fail right off the bat because they use absolute /ota paths.

Should be a relatively straightforward fix in the JS. Happy to submit a PR given the JS source you'd like to see modified.