add stage to normalise certificate white space

[iay]

The sdss-xalan-md package includes a whitespace wrapper for use with X.509 certificate data embedded in metadata (uk.ac.sdss.xalan.md.TextUtils#wrapBase64). This is used with the MDA in the clean-import.xsl transform as part of our main toolchain, and in import.xsl. It is also used in extract-authorities.xsl, but that's nearly defunct now.

This relies on the use of Xalan as an XSLT processor, and will not work with others. We should turn this functionality into an MDA stage to reduce this dependency.