Render pdfs embedded in issues

Allow to render dropped PDFs in issue to be directly displayed embedded.

Similar how

• [image.png](/uploads/ddc53f711e0055090f5699af0fcc5f15/image.png) links to the uploaded file

and

• ![image.png](/uploads/ddc53f711e0055090f5699af0fcc5f15/image.png) directly embeds the image,

![document.pdf](/uploads/ddc53f711e0055090f5699af0fcc5f15/document.pdf) should directly render the pdf.

Direct rendering of PDFs is already implemented for the repository browser (gitlab-foss!10419 (merged)).