[5.7] Don't print the generated application key

[SjorsO]

The artisan key:generate command prints the generated application key in the console. Since the application key should be secret, it might be a better idea to not print the key. That way there is no chance it will end up in console history or a log file.

This PR makes the artisan key:generate command not print the generated key.

[laurencei]

But if you have access to the console (to see the history) - then wouldnt you have access to the .env file, and thus the key anyway?

[deleugpn]

For me, that was a cool feature. All I had to do was php artisan key:generate and copy from the terminal to place it where I wanted it.

[SjorsO]

If you want to generate a key and only have it printed in your console, you can run:
artisan key:generate --show