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.

app/models/application_record.rb

@@ -1,7 +1,13 @@
 class ApplicationRecord < ActiveRecord::Base
   self.abstract_class = true
 
-  include Danbooru::Paginator::ActiveRecordExtension
+  concerning :PaginationMethods do
+    class_methods do
+      def paginate(*options)
+        extending(PaginationExtension).paginate(*options)
+      end
+    end
+  end
 
   concerning :SearchMethods do
     class_methods do