Implementation of returning modules

Return only the module if the slug contains a module slug - that is the
slug is of the form /<dashboard-slug>/<module-slug> - ie a dashboard has
been found for <dashboard-slug> and there is a remaining part of the url
that corresponds to a slug in the dashboards modules.
Add logic for finding tabs when specified

Also refactored the module finding logic as some of it was similar to
tab finding logic
Move logic from view to model

Further changes might use the presenter pattern, but for now it is good
to have all the spotlightify logic in one place.
Merge pull request #231 from alphagov/update-slug-for-modules-and-tabs

Add modules and tabs logic to dashboard view