Heroku's attempts to run collectstatic automatically but Baby Buddy never
wants that. Instead of offering a user-editable environment variable this
change uses Heroku's secret generator to set the variable so the user cannot
change it during deploy.
The previous setting of "Etc/UTC" appears to no longer be valid in certain
circumstances. This was causes tests to fail on user settings because the
provided default was invalid. "UTC" should be more uniformly recognized so
hopefully this does not have any downstream effects.
Closes#203