Rename /category/xyz paths to /c/xyz -- @SamSaffron did most of the

work even though I'm merging the patch!

test/javascripts/integration/topic-discovery-test.js.es6

@@ -7,7 +7,7 @@ test("Visit Discovery Pages", function() {
     ok(exists('.topic-list .topic-list-item'), "has topics");
   });
 
-  visit("/category/bug");
+  visit("/c/bug");
   andThen(function() {
     ok(exists(".topic-list"), "The list of topics was rendered");
     ok(exists('.topic-list .topic-list-item'), "has topics");