Replace the conflict page with a two column version
This is the first version of the two column edit conflict page. A new
class is introduced extending the EditPage to show the two column version.
Basically this version moves the editor with the conflicting change from
the bottom to the top into a column next to the editor with the version
that will be saved when the conflict is solved.
Next steps will be addressed in follow-up patches.
Most parts of the code if the original EditPage are still used to avoid
interferences with other bits of the edit page logic.