Fix broken assert in MergeParagraph op

The operation was attempting to assert that the source step was the
first step within the paragraph as well. Instead, it was only asserting
that the source paragraph was adjacent to the destination paragraph.
Add unit tests showing MergeParagraph OT issue
fixup! Add unit tests showing MergeParagraph OT issue
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.
fixup! Fix OT when two adjacent paragraphs are merged backwards