Allow importing pages as subpage

Allow importing pages as subpage

Adding a "root page" input to Special:Import, which is used as prefix
before each imported page. With this option, it is possible to import
pages as subpage of a user or a project page.

On de.wp the import is often done into a low used namespace (100 or 101)
and than moved to the user space. Doing this in one step, makes things
easier.

Change-Id: Id5dbf742295a1bbddd8cb6eaa09fb28051f26613