Markdown in print documents

I implement converting Markdown content in invoicing transactions to
HTML and storing it in the built XML code.  Then I add the html.xsl file
which converts the embedded HTML code into FO objects.

Furthermore, I rework all XSL code and make it more general by using
named templates and variables.

At last, I fix a bug that prevents generating XML for print documents
because I cannot access the foreign key property "reminders" in User
objects.