Editconflicts

In this patch, edit conflicts are automatically merged when possible. If there is a merge conflict, the user is notified. (This looks ugly, but can and should be prettified).

After migrating, old data need to be updated with parent and child references in ArticleContent, or things like history might stop working.

It might be an idea to fix a few more things before merging this into master.

This branch fixes #27.