Use VE's fixBase utility so link previews point to the right place

This'll prevent link previews from pointing to URLs like
/w/Main_Page if you happen to be at a /w/index.php?...-style URL.

Depends on I42dfd4b90 in VisualEditor.

Bug: T96492
Change-Id: I6da5de2d94ce9d8c7b4a051e76a0937744800259