Make ESPWA compatible with being hosted as a web_server front end

[DanielBaulig]

web_server has the option of specifying a JS and CSS file to load its own locally "hosted" front end from. I should look into making ESPWA compatible with this, such that either ESPWA loads (indirectly) from the device or (probably preferable) the device redirects to ESPWA adding the device via the query parameter URL