Generate APP_SECRET on create-project

[rosier]

The current demo v2.7.0 doesn't work out-of-the-box anymore for new installs since the value for APP_SECRET was removed from .env (74d38a5).

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 propose 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.

Also .env.local is ignored by git by default and prevents contributors from accidentally committing a value for APP_SECRET to this repository.

[javiereguiluz]

Thanks Jan. This is technically correct and I appreciate the effort to fix this bug. But, as explained in #1577, I think this solution could fail when running Composer with the --no-scripts option. So, I propose to close this PR in favor of the other one. Thanks.