Change notes for front end users

This PR contains a number of features needed for users to see Change Notes on manuals

- a placeholder JSON for the EIM updates
- a link to the updates from the header
- refactoring the ManualsController to fetch the Manual from a before_filter
- adding the route, action and view for updates
- adding a ChangeNotesPresenter which takes the flat updates JSON array and organise it by Year, Day, Document and Change
- matching the markup of show.html.erb in updates.html.erb as much as possible
It would probably be sensible to push this into the `ManualPresenter` and call `@manual.updates_url`