Combine navigation

The logic is that when there is no search term, clicking University or Amenities returns you to the categories view (aka "browse), otherwise run the query again with the new type (ie /university or /amenities).
However, when you have a result for a "University" search, clicking Amenities returns you to the categories view rather than re-running the query. This is to meet the use case "I want to find atms near Somerville college": find Somerville College, click Amenities, click atms. This solution is not ideal as we probably want something more like how the transport options work for all amenities (which calls into question pull request #48). See comment on issue #40.