Option to set items per page, on a per model basis, v2

This builds on the previous PR for a global override of the number of items per page, and allows it to be provided on a per-model basis.

For example in app/upmin/models/admin_shipment.rb:

```ruby
class AdminShipment < Upmin::Model
  items_per_page 20
end
```
If not specified, or no custom class exisits, then either the global override `config.items_per_page` if set, or the builtin default will be used.