Fixed #20081 -- Minimized risk of SECRET_KEY leak.

By default set the SECRET_KEY value to be loaded from env vars.  If not
set and DEBUG==True generate a random SECRET_KEY.  Also adds the
generatesecret management command, which prints a random secret key to
stdout.
I'm not sure.  My initial thought is that the entire file, whitespace and all should be set as the `SECRET_KEY`.  However, I do see how the current implementation could lead to unexpected results.  I'd like to see if anyone else has thoughts on this.