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).

app/models/user.rb

@@ -380,6 +380,16 @@ class User < ApplicationRecord
         end
       end
 
+      def page_limit(level)
+        if level >= User::Levels::PLATINUM
+          5000
+        elsif level == User::Levels::GOLD
+          2000
+        else
+          1000
+        end
+      end
+
       def tag_query_limit(level)
         if level >= User::Levels::BUILDER
           Float::INFINITY
@@ -484,6 +494,10 @@ class User < ApplicationRecord
       @upload_limit ||= UploadLimit.new(self)
     end
 
+    def page_limit
+      User.page_limit(level)
+    end
+
     def tag_query_limit
       User.tag_query_limit(level)
     end