Refactor to extend TextConflictHelper instead of EditPage
This allows TwoColConflict to continue working even if another extension
has overridden the EditPage instance.
Bug: T176393
Depends-On: Ie415edd84329c02d5762477f8a171fced85b01a2
Change-Id: I651691a40febc1e8c8049f6a590d0e003b0550ed