pagination: refactor to avoid counting pages in API.

Previously the page-based (numbered) paginator would always count the
total_pages, even in API calls when it wasn't needed. This could be very
slow in some cases. Refactor so that total_pages isn't calculated unless
it's called.

While we're at it, refactor to condense all the sequential vs. numbered
pagination logic into one module. This incidentally fixes a couple more
bugs:

* "page=b0" returned all pages rather than nothing.
* Bad parameters like "page=blaha123" and "page=a123blah" were accepted.

test/unit/post_sets/intro_test.rb

@@ -1,5 +1,4 @@
 require 'test_helper'
-require "danbooru/paginator/pagination_error"
 
 module PostSets
   class IntroTest < ActiveSupport::TestCase

test/unit/post_sets/post_test.rb

@@ -1,5 +1,4 @@
 require 'test_helper'
-require "danbooru/paginator/pagination_error"
 
 module PostSets
   class PostTest < ActiveSupport::TestCase
@@ -86,7 +85,7 @@ module PostSets
         end
 
         should "fail" do
-          assert_raises(Danbooru::Paginator::PaginationError) do
+          assert_raises(PaginationExtension::PaginationError) do
             @set.posts
           end
         end