Cleanup unused image files

We no longer show screenshots on the pages of exempt organisations so these
images are redundant.
Allowable filetypes to support transition

Allowable filetypes to support department content transition; diff,
chm, sch, wsdl, xslt
Reorder filetypes

Reorder filetype lists alphabetically
Remove title separator line for freedom of information addresses
Merge pull request #1844 from alphagov/freedom-of-information-title-separtor

Remove title separator line for freedom of information addresses
Merge pull request #1845 from alphagov/hmrc-whitelisted-filetype-additions

Whitelisted filetype additions for department transition
Merge pull request #1842 from alphagov/cleanup-unused-images

Cleanup unused image files
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.