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 diasllow it.
Merge pull request #1843 from alphagov/timeout-for-pdf-thumbnail-generation

Timeout of 10s for PDF thumbnail generation