Separate 'register' and 'register backend' tasks

In the future, we will want the 'register' task to register both the 
backend and the browse pages. To prepare for this, separate the backend
registration into a separate task, following the convention in the 
Frontend app.
Register the browse routes with the router

This adds a new Rake task to register the '/browse' and '/browse.json'
routes to the collections app. 

At the present time, we do not want this to run automatically, so this 
remains a standalone Rake task that must be run manually in the preview
environment.
Unregister the existing browse redirect routes

At present `/browse/business` and `/browse/visas-immigration` redirect 
to their special-case counterparts. This is going away, and we'll be 
removing these routes from router-data soon. 

Until then, we need to ensure that these aren't registered whenever we 
register the mainstream browse routes.
Merge pull request #12 from alphagov/register-mainstream-browse-route

Register mainstream browse route