Revert "Moved VersionCode to DataSets"

This reverts commit cb9dd634bbaeba70912aae127f92535d1a851225.
Moved VersionCode to DataSets and Deleted ResultVersions

It seems that versioning is really only important when a dataset gets
“published” or made public, whereas revisions to Results are likely not
worth tracking. Therefore, I:
* Deleted ResultVersions table
* Added VersionCode and VersionOfDataSetID to DataSets table.
Is a VersionExplanation necessary for a Dataset? Can that function be
fulfilled by the DataSetAbstract?