404 if the Repo can't find a document

With the introduction of a Repo for fetching documents, the Repo would
throw an error if params[:section_id] didn't match a placeholder file
instead of allowing the controller to respond with a 404. This commit
checks for the existence of a file first, which if false allows the
controller to respond with a 404.
Add cucumber to manuals-frontend.

Gem versions are pessimistically fixed to match those in other projects
(rspec 2 rather than 3, for example).

Also include two passing tests covering local manual and section
browsing and the fix introduced in 465ef06.
Update language used to match the rest of the spec
Don't rely on specific classes for subsections.

By genericising the cucumber feature to just 'click on <x>' rather than
'click on the <x> subsection' we don't lose any clarity, but we remove
reliance on specific classes (i.e. the previous 'within' block to detect
where on the page to click)
Fix /guidance redirect route

Routes containing a query string are not valid redirect targets for the
Router API. The publications index is an even worse option than the
pre-filtered list, but it's the best we have until we build a proper
index page.
Adding placeholder JSON for EIM updates
Refactor Manual fetching to before_filter
Adding route and controller action for #updates
Rendering updates view

Adding a link to the updates from the header and an updates view. The
ChangeNotesPresenter takes the updates JSON and organises it by Year,
then by Day, then by Document.