nuxeo-cli update after Polymer 3 migration

After Polymer 3 migration there is a need to update nuxeo-cli in order to fetch the correct dependencies, to use npm only and also to reflect to repo merge on nuxeo-elements.

More details can be found on the related tickets, despite having a lot of small task tickets, the most important information should be on the following: ELEMENTS-862, NXP-26766, NXP-26767, NXP-26830, ELEMENTS-876 and NXP-26845.

In summary:

• nuxeo-elements is now a monorepo that contains core, dataviz, storybook and ui;
• nuxeo-web-ui also contains all the UI contributions for the addons (e.g. easyshare, live connect, etc.);
• bower is not used anymore and dependencies were migrated to npm;
• all the elements are using Polymer 3;