Make sure that Workspaces related menus don't affect subwiki local users

When the Workspace feature is installed but the user chooses to create regular subwikis with the WikiManager module, we need to ensure that the Workspace menus do not bother the local users of subwikis.

Noticed menu problems for local subwiki users:

• extra menu separator before the Wiki menu
• extra workspaces section in the User submenu