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.
This patch needs some refactoring in the EditPage class. To use it you need
to checkout https://gerrit.wikimedia.org/r/#/c/318535/ on your mediawiki