Feature/63 complete checkbox button for marking activities complete

https://trello.com/c/OS8EIPsD/63-complete-checkbox-button-for-marking-activities-complete
Use a before_create to set this since it should be up to the model/business layer to set this timestamp not the controller.

I used to use a gem for setting defaults in AR called `default_value_for` but for a while it wasn't working with newest version of Rails. Looks like they fixed that. https://github.com/FooBarWidget/default_value_for