Use correct values when sorting by has_many associations

[sinsoku]

When sorting by has_many associations, Administrate assumes that the table and column names are #{attribute}.id.
An error will occur if the user defines a different association than the table name or uses a primary key other than :id.

For example, the following code causes an error.

# db/migrate/20201016000000_create_users.rb
create_table :users, primary_key: "uid" do |t|
  t.references :company

  t.timestamps
end

# app/models/company.rb
class Company < ApplicationRecord
  has_many :employee, class_name: "User"
end

Administrate::Order.new("employee", "asc").apply(Company.all).take
#  Company Load (0.5ms)  SELECT "companies".* FROM "companies" LEFT OUTER JOIN "users" ON "users"."company_id" = "companies"."id" GROUP BY "companies"."id" ORDER BY COUNT(employee.id) asc LIMIT ?  [["LIMIT", 1]]
# Traceback (most recent call last):
#        1: from (irb):1
# ActiveRecord::StatementInvalid (SQLite3::SQLException: no such column: employee.id)

This commit fixes Administrate::Order to use the correct table and column names.

[pablobm]

Thank you for this PR! It looks good to me 👍

[sinsoku]

I fixed conflicts and force-pushed.