Fix an error in a documented lookup example.
Fixed #22675 -- makemigrations --dry-run to output migrations to stdout.

`makemigrations --dry-run` will output the complete migrations file
that would be written if it's used along with `--verbosity 3`.
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.
fixed typo.  Thanks Pentusha.