Add after_{create,update,delete}_commit callbacks to observers

[pelargir]

Rails 5 adds after_{create,update,delete}_commit callback aliases. See: https://blog.bigbinary.com/2016/05/11/rails-5-adds-after_create-aliases.html

This PR makes these callbacks available in observers. Includes tests to verify the callbacks were added properly. This also maintains backwards compatibility with Rails 4.

[cprodhomme]

need this 🙏