Generate the Samples Report PDF on the backend #1843
Description
The samples report PDF in #1840 relies on JavaScript libraries on the frontend, which is different from how we generate PDF elsewhere on the system. Ideally we shouldn't introduce many ways to do the same thing.
The only bothering thing is that the charts are generated on the frontend using d3 (which we already used to generate other charts). Ideally we'd generate them on the backend, but I can't find any good Ruby library to generate SVG. Running d3 on the backend feels weird, and it doesn't work with MiniRacer (I get some missing JS references for d3).
Follow up of #1840