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.
Similarly, `'links to that site\'s hits category path with the category and period'` would be more accurate here now.