Integrate esp-web-flasher to install firmware

[opatut]

In the simplest form we provide a little UI to flash the USB device with the standard firmware. We can follow the logic of esp-web-tools, but use the same library esp-web-flasher directly, building a new UI with our UI toolkit directly into the frontend. That won't be much work.

For the flashing content, we can then have the API download the latest firmware images from github and re-expose them (to circumvent CORS issues). Possibly allow choosing the firmware version to install.

In the future, we can also go crazy here:

• Have the API generate a custom SPIFFS image for flashing configuration directly from the Web UI, alongside the firmware or after flashing the firmware, containing the api key, api url, and potentially wifi credentials. See this gist I made a while ago for generating a SPIFFS image in Python.
• Allow the portal instance maintainers to
  • enable or disable parts of these features
  • provide a preset for certain configuration options
  • pin the firmware version or choose a custom image with support for other features maybe
• Integrate improv-wifi.