Migrate webmin API + UI into duniter-ui project

@Insoleet demonstrated at https://github.com/Insoleet/duniter-ui/tree/specialized-node that we could externalize the Webmin API + UI into a specialized node.

This is wonderful idea, because we can now remove all the webmin code from duniter/duniter and place it into duniter/duniter-ui, making the core code simpler.

This also implies to change the way we produce a release, since the UI won't be in dunter/duniter anymore. We have to think about:

• desktop version
• server version (how to have the UI?)