Webapps have static entry point of 'public'

[tonybentley]

Webapp assumes location for static index.html will be ${projectRoot}/public/

Reference: https://github.com/SignalK/signalk-server/blob/master/src/interfaces/webapps.js#L54

When using create-react-app the default source file location is public/ and build location is build/ , which requires the developer to eject from the default React app convention and configure source and build directories to follow Signalk convention.

Solution: Make the webAppPath appended directory to be configurable within the plugin.