Replace database diagrams with DITAA diagrams in the Asciidoc

[andyjefferson]

This would use the Asciidoctor-diagram plugin as per http://asciidoctor.org/docs/asciidoctor-diagram/
and the diagrams would be in DITAA format, following this http://ditaa.sourceforge.net/

While this works fine for embedding in the HTML output, it doesn't currently work for the PDF generation.
This is because the asciidoctor diagram will generate PNGs of the DITAA diagram in the target directory, but the PDF process looks in the source directory only currently. We don't want to generate images into the source directory since they would get checked in! See the following demo repo that shows the problem

https://github.com/andyjefferson/asciidoctor-pdf-diagram-combo