Fix internationalization of model names in nested_has_many buttons

[atzzCokeK]

Problem

When using non-English locales, the model names in the "Add" and "Remove" buttons are not correctly translated. The current implementation uses field.associated_class_name.titleize directly, which doesn't go through the Rails translation system.

For example, even when I have defined translations for model names in config/locales/ja.yml:

ja:
  activerecord:
    models:
      app_release: "お知らせ"

The button still displays "Add App Release" instead of "Add お知らせ".

Steps to Reproduce

1. Set up a non-English locale (e.g., Japanese)
2. Define translations for model names in the locale files
3. Use nested_has_many fields in a form
4. Observe that the model name in the buttons is not translated

Proposed Solution

I propose two possible solutions:

1. Prioritize the name_label option if it's present:

<%= link_to_add_association(
  field.options.try(:[], :name_label).present? ? 
    "#{I18n.t('administrate.fields.nested_has_many.add_with_name', name: field.options[:name_label])}" : 
    I18n.t("administrate.fields.nested_has_many.add", resource: field.associated_class_name.titleize),
  f,
  field.association_name,
  ...
) %>

2. Or use Rails' translation system to get the model name:

<%= link_to_add_association(
  I18n.t("administrate.fields.nested_has_many.add", resource: I18n.t("activerecord.models.#{field.associated_class_name.underscore}", default: field.associated_class_name.titleize)),
  f,
  field.association_name,
  ...
) %>

I'm happy to submit a pull request with this change if it's considered valuable.