pagination: refactor page limits.

Refactor page limits to a) be explicitly listed in the User class (not
hidden away in the Danbooru config) and b) explicitly depend on the
CurrentUser (not implicitly by way of Danbooru.config.max_numbered_pages).

config/danbooru_default_config.rb

@@ -107,11 +107,6 @@ module Danbooru
       2
     end
 
-    # After this many pages, the paginator will switch to sequential mode.
-    def max_numbered_pages
-      1_000
-    end
-
     # Maximum size of an upload. If you change this, you must also change
     # `client_max_body_size` in your nginx.conf.
     def max_file_size