This change re-introduces linked images in HTML export
but keeps embedded image export in mailmerge e-Mail function

Change-Id: I9e956b590f4c675b5954e19e4d3948c36b55f1e6
Reviewed-on: https://gerrit.libreoffice.org/23359Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Oliver Specht <oliver.specht@cib.de>