Create layouts for UserWorkspaces

Steps to reproduce:

1. Log in to Nuxeo Web UI
2. Navigate to the user workspace
3. In the breadcrumbs, click on the text "UserWorkspaces"
4. Observe that you navigate vers a page which displays an error

The error is due to the fact that no View layout has been designed for the type UserWorkspaceRoot.

Expected behavior: as in JSF UI, the text "UserWorkspaces" should be disabled in the breadcrumbs, as this type has the facet HiddenInNavigation

Another acceptable solution would be to write the needed layouts for UserWorkspaceRoot, as it's sometimes useful to list the user workspaces (to do some cleanup for example)