Optimize image assets with ImageOptim
Revert "Optimize image assets with ImageOptim"

This reverts commit 9e0d4a6b60e68496e0f5e087e19a984ea8d97182.
Optimise assets that are not autogenerated
Merge branch 'master' of github.com:guardian/frontend into optimise-image-assets
CSS references the correct md5 of modified assets
Merge pull request #1167 from guardian/optimise-image-assets

Optimise image assets with ImageOptim