Change default top_level_container_name for itsisu to be "activity" (as it is on pro)

In config/initializers/01_load_config.rb on line 25 we try to set the class for the top-level container.( http://bit.ly/1jRCAK6 )

The default configuration for itsi sets the TOP_LEVEL_CONTAINER_NAME to "Unit" -- but ::Unit is not an actual class name, so I keep getting `uninitialized constant Unit`

On production this is set to "Activity".