PDF generation fails on Scientific Programme once a relative URL is used

[vaclavstepan]

Describe the bug
In the Scientific programme, the tracks are described in Markdown. When a relative URL is used for an included image, using a HTML img tag, generation of a PDF version of the document fails with:

<img src="/event/2/images/12-MariaZankl-small.png"/></para>' caused exception fileName='/event/2/images/12-MariaZankl-small.png' identity=[ImageReader@0x7fed8f2fbdc0 filename='/event/2/images/12-MariaZankl-small.png'] Cannot open resource "/event/2/images/12-MariaZankl-small.png"

You only find this, when you try to generate the PDF for the Scientific Programme page, in HTML view it shows fine.

To Reproduce
Steps to reproduce the behavior:

1. Go to Management area
2. Upload an image and copy its URL somewhere
3. Add an img tag as follows into the description of some of a track in the Programme part of the Management area
4. In the Display part, go to the Scientific Programme page and click the PDF icon on the top left of the page.
5. See error

Expected behavior
It should accept the relative URL.