Timeout of 10s for PDF thumbnail generation

www.agileplannerapp.com/boards/173808/cards/8160
fixes: https://govuk.zendesk.com/tickets/868919

we show the fallback pdf thumbnail image instead of a generated one if the thumbnail generation takes longer than 10s. better to process and upload without a thumbnail, than to disallow it.

also renamed a constant to have a more suitable name.