Create class_name option on has_many fields

Created by: gracewashere

Upcase has the following code:

class Mentor < ActiveRecord::Base
  # ...
  has_many :mentees, class_name: 'User', foreign_key: 'mentor_id'
  # ...
end

Before this commit, Administrate incorrectly tries to look up a MenteeDashboard instead of a UserDashboard to figure out how to display the association table.

This commit allows developers to specify the class_name that will be used to look up the proper dashboard for a has_many association.

The option is used like this:

  def attribute_types
    {
      # ...
      mentees: Field::HasMany.with_options(class_name: "User"),
      # ...
    }
  end

Background: https://trello.com/c/kypYTxFt