Update the view per new db schema

We now store each individual time a given slide has been shown so the logic needs to be updated. We still need to add a way to view more detailed statistics...
Move display histories to new controller

We need a way to show more detailed history information for slides shown and a clean way to reset the history, so it clearly represents a nested resource.
Tidy up navigation links

We now check if @display exists on the main navigation partial, leading to way more DRY code.
Allow clearing of display history
Add tests for the new history controller