Fix choosing a date from category hits graph pages

If you were viewing the hits in a category, for example:

/sites/businesslink/hits/category?category=errors

The code was overwriting the querystring and so omitting the category
parameter. Following that link would trigger a 5xx error because category was
empty when it shouldn't be.

Using the URI library, we can add or overwrite just the relevant part of the
querystring.
Merge pull request #397 from alphagov/fix_hits_graph_period_selection_from_category_pages

Fix choosing a date from category hits graph pages