Fix tree navigation display when user workspaces are involved

Here are some scenarii which lead to errors or inconsistency in the display of the navigation tree

1. From the dashboard, follow a link to a document in your personal workspace => error if never browsed your personal workspace before
2. From the dashboard, in my tasks gadget, follow a link to a document in a user personal workspace => error or displaying your personal workspace tree
3. As an administrator, go to your personal workspace,
   then up one level using the breadcrumb => still displaying your workspace tree ; it should display nothing or the UserWorkspaces tree
   then go to another workspace => still displaying your workspace tree ; it should display this user workspace tree