wp_enqueue_script not loading my custom js file

You need to reference your WordPress template directory when you register the script.

Change this:

wp_enqueue_script('my_javascript_file', '/javascripts/app.js', array('jquery'));

…to this:

wp_enqueue_script('my_javascript_file', get_template_directory_uri() . '/javascripts/app.js', array('jquery'));

Codex reference: get_template_directory_uri()