SOC-2753 | Fix several issues while creating new page in space

Fix description:
 - SPACE_URL should be computed automatically depending on the context. Remove this field from the portlet preference to avoid editing by the user.
 - When the user try to add space-context portlet (Space Menu Portlet, Space Activity Stream Portlet, Space Setting Portlet) in a page which is not in a space, the portlet should display warning message to inform that this is out of space context.