Use Ghostscript to generate PDF thumbnails

ImageMagick cannot convert PDF to images by itself and instead delegates
to Ghostscript. By bypassing ImageMagick and going to Ghostscript
directly, we see a big improvement in thumbnail generation speed:

	Test file 1 with ImageMagick: 30.7 seconds
	            with Ghostscript:  1.5 seconds
	Test file 2 with ImageMagick: 30.7 seconds
	            with Ghostscript:  1.2 seconds
	Test file 3 with ImageMagick: 31.4 seconds
	            with Ghostscript:  1.7 seconds
Merge pull request #1645 from alphagov/faster-pdf-thumbnailing

Use Ghostscript to generate PDF thumbnails