Get rid of decent_exposure gem

The use of the `decent_exposure` gem gave us very little real benefit, adds an extra external dependency and a further DSL for developers coming onto the project to learn. This removes it in favour of more idiomatic Rails code. In most instances, the code reverts back to straightforward instance variables shared between controller actions and views. In cases where the object in question is accessed more widely than a couple of views (e.g. all frontend pages expect an Organisation to be present), access to the object via an explicit helper method has been maintained.