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

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 `config/initializers/upmin_admin.rb` add:

```ruby
Upmin.configure do |config|
  config.items_per_page = {user: 10, order: 40, default: 20}
end
```
Any unamed models will take either the built-in default of 30, or :default if provided (:default is optional).

If only the default needs to be overridden, then as a shortcut it can be provided as an integer: `config.items_per_page = 20`