Fix hits pagination links

I think these have been broken since we upgraded to Rails 4.x and the way that
these routes were implemented was no longer supported.

This way is less code, less metaprogramming and easier to follow.

This bug drove me nuts when trying to add missing mappings to Directgov, I assume it does the same for other users.
I think this needs to be `category_site_hits_path` - at the moment the 'See all errors' etc links on a site's summary page link to the universal analytics category pages.