Always include WebPreferences when exporting a non-terminal page

Use case:

• Create page A.B.WebHome
• Set some permissions on page B.WebHome(this created B.WebPreferences)
• Export A.B.WebHome to XAR
• Delete space A.B (with children)
• Import XAR and verify that the permissions are not set anymore (the reason is because the WebPreferences page was never exported)

From an end user POV this is awkward and a solution is to always export the WebPreferences page when exporting a non-terminal page.