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.

config/initializers/delayed_jobs.rb

@@ -1,5 +1,4 @@
 require 'delayed/plugin'
-require 'danbooru/paginator/active_record_extension'
 
 class DelayedJobTimeoutPlugin < ::Delayed::Plugin
   callbacks do |lifecycle|
@@ -13,4 +12,3 @@ Delayed::Worker.logger = Logger.new(STDOUT, level: :debug)
 Delayed::Worker.default_queue_name = "default"
 Delayed::Worker.destroy_failed_jobs = false
 Delayed::Worker.plugins << DelayedJobTimeoutPlugin
-Delayed::Job.include(Danbooru::Paginator::ActiveRecordExtension)