Generate APP_SECRET on create-project

[rosier]

The current demo v2.7.0 doesn't work out-of-the-box anymore for new installs.

Running the following commands results in the error: "A non-empty secret is required."

symfony new --demo sfdemo
cd sfdemo
bin/console doctrine:schema:validate

I purpose to generate APP_SECRET on composer create-project to improve the UX for new installs.

I opted to create a .env.local file because it works for both the production and the development environment.