Use environment variables for app config

[aschempp]

This allows to use .env files for the application configuration, as is best practice in Symfony.

FYI

• If the env variable is not set, the old parameters are still applied, so there should be full BC
• That also means the install tool does not need to be updated
• The Contao API already provides support for reading + writing the .env variables, so the Manager would already be able to configure the database connection 😎

[leofeyer]

If we have a database URL, we do not need the database_* parameters anymore, do we?

[aschempp]

we need them for backwards compatibility and the install tool

[aschempp]

Btw this also means we no longer need to re-compile the container when changing database credentials (which does not apply to the install tool yet though)! 😎

[leofeyer]

This concept does not work for the mailer URL though:

A string value must be composed of strings and/or numbers, but found parameter "mailer_user" of type NULL inside string value "%mailer_transport%://%mailer_host%:%mailer_port%?username=%mailer_user%&password=%mailer_password%&encryption=%mailer_encryption%"

[leofeyer]

Ah, it does not work for the database URL, either:

A string value must be composed of strings and/or numbers, but found parameter "database_user" of type NULL inside string value "mysql://%database_user%:%database_password%@%database_host%:%database_port%/%database_name%"

[aschempp]

The mailer might be an issue because some parameters are optional. But that's not the case for the database, how can database_user be NULL for you?

[leofeyer]

See #768.