Collection cache max age should be 5m

www.agileplannerapp.com/boards/173808/cards/8023

HMRC need to publish a collection page which needs
to reflect the latest changes in no longer than 5m.

made a change to the `CacheControlHelper` to allow
overriding the `cache_max_age` from the controller,
so that we can set a max age of 5m for collections.
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
Merge pull request #1837 from alphagov/change-collections-max-age

Collection cache max age should be 5m