Add i18n support for Field::HasMany (#2114)

If you have ActiveRecord i18n translation of the model you don't need to do
anything extra, if not, make an ActiveRecord i18n translation of the model.

e.g.:

The recipe has many recipe items.

```ruby
ATTRIBUTE_TYPES = {
  recipe_items: Field::HasMany,
  id: Field::Number,
  name: Field::String,
  package_spec: Field::String,
  remark: Field::String,
  created_at: Field::DateTime,
  updated_at: Field::DateTime,
}.freeze
```

```yaml
zh-TW:
  activerecord:
    models:
      recipe: 配方表
      recipe_item: 材料
```

app/views/fields/has_many/_index.html.erb

@@ -16,4 +16,4 @@ as a count of how many objects are associated through the relationship.
 [1]: http://www.rubydoc.info/gems/administrate/Administrate/Field/HasMany
 %>
 
-<%= pluralize(field.data.size, field.attribute.to_s.humanize.downcase.singularize) %>
+<%= pluralize(field.data.size, t("activerecord.models.#{field.attribute.to_s.singularize}", default: field.attribute.to_s.humanize.downcase.singularize, count: field.data.size)) %>