Relay custom paths to the JavaScript

[thisisaaronland]

Uncertain how best to do this right now since the web application uses an io/http.FS instance rather than Go templates.

An easy approach would be to store/load Go templates using an io/http.FS instance. That would mean the HTML+JS web application (defined in www) would stop being a separate (portable) artifact but maybe that is an acceptable trade-off.

Note: Until Go 1.16 is released, this issue is specific to the www branch.