Added 'free' to list of validated stage types

QA plan (test on staging, requires newer sport_admin changes deployed there):
* In Divisions > Divisions, click on a division.
* Click the button to add a new stage, and select "Custom Games" in the popover's Stage Type dropdown.
* Enter a number for max teams and click Add Stage.
* The add should be successful and no errors should occur as a result of adding the stage.

Unit testing:
* Run `npm test` in the ngin_client directory. Five new tests should show under Flight Stages, one for each flight stage type. All of them should pass.