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/controllers/delayed_jobs_controller.rb

@@ -3,7 +3,7 @@ class DelayedJobsController < ApplicationController
   before_action :admin_only, except: [:index]
 
   def index
-    @delayed_jobs = Delayed::Job.order("run_at asc").paginate(params[:page], :limit => params[:limit])
+    @delayed_jobs = Delayed::Job.order("run_at asc").extending(PaginationExtension).paginate(params[:page], :limit => params[:limit])
     respond_with(@delayed_jobs)
   end