Order by belongs_to field

Created by: sejinkim1904

What does this PR do?

• Updates Order to be able to sort by belongs_to field
  • Updates initialize to take in association_attribute
  • Adds order_by_association_attribute
  • Adds order_by_attribute
  • Updates order_by_association to use order_by_attribute for belongs_to associations
  • Adds association_has_attribute?
• Adds belongs_to_sorting_field to ApplicationController to be overridden by users to select which field to sort by
• Adds belongs_to_sorting_attribute to OrdersController for testing
• Updates/adds tests to specs
• Adds Customizing belongs_to Sorting Field section in /docs/customizing_controller_actions.md

How should this be manually tested?

• Start local server
• Visit /admin/orders
• Click on 'Customer'

Any background context you want to provide?

My team currently uses administrate in our application and we noticed when trying to sort by a belongs_to association, by default it sorts by the association's id. We found a workaround by sub-classing Administrate::Order in our controller to be able to sort by an association's field. Our implementation was very specific to our data model but I thought this feature could be useful for future or current devs so I decided to create a more dynamic version.

Screenshots (if appropriate)

Before:

class OrdersController < Admin::ApplicationController
end

After:

class OrdersController < Admin::ApplicationController
  def belongs_to_sorting_attribute
    :name
  end
end

Questions:

• Do Migrations Need to be ran? NO
• Do Environment Variables need to be set? NO
• Any other deploy steps? NO