Fix/minor security issue

Good idea to remove grunt-exorcise. It's intended to extract the map from a compiled JS file and move it to a standalone .map file, so that we don't use up people's bandwith serving them .map data they don't need. But it's stopped being updated, and there doesn't seem to be any obvious equivalent that is up to date (and thus will deal with the security alert). It might be worth recompiling the JS as well (yarn run grunt) to produce new compiled JS that reflects that change.

It looks like the .idea/ files still need removing, even after the change to the .gitignore - I think because that makes git ignore any files added to the .idea/ folder in future, but not to ignore ones that are already committed.

That's done, thanks :)

dxw / govuk-theme

Fix/minor security issue #19