Do not use maven to build Web UI

Currently, we rely on maven to build Web UI. However, this is an abuse, since we are just deploying web resources. Furthermore, we need to constantly update the parent pom version whenever there is a server release, which defeats the purpose of having independent release life cycles.

We have a couple of challenges here:

1. Building a package without maven. We can use a [tool|https://github.com/nelsonsilva/nuxeo-tools/blob/master/lib/package.js] developed by Nelson a few years ago.
2. We still rely on nuxeo-ftest to run the tests, so we need to find an alternative for this.