Fix OT when two adjacent paragraphs are merged backwards

Transforms were not ensuring a consistent style name was applied to the merged paragraph on all clients.

Incidentally... this issue was found when playing with the split screen editor!
Phew... done. I've fixed extended the assert in ``OpMergeParagraph`` to properly verify it's at the first step, added a second test so both new conditions are tested, and then corrected the transform. Hopefully it's all magically wonderful now :flushed: