ISIS-714, ISIS-788: further fixes for blobs and clobs in DataNucleus.

- for ISIS-714, the clob/blob stuff was still broken
   - improve the IsisClobPanel, new implemenation of Wicket's IResource (CharSequenceResource) so preserve name and media-type.
- for ISIS-788, removed the redundant Isis{Blob/Clob}Converter classes