Investigate Edge compatibility

[glynhudson]

Sean Fosberry has reported that the web page does not load on Microsoft Edge:

https://community.openenergymonitor.org/t/openevse-with-wifi-build-v2-and-testing/4993/13?u=glyn.hudson

[jeremypoulter]

Fundamentally this is caused by me-no-dev/ESPAsyncWebServer#94 so will need to fix this to enable full support in Edge, however #99 fixes enough to make the UI usable in Edge/IE