[RFC] Bypass db connection setup when using env component

[sheeep]

Bypass the database connection setup per form if the application is already configured using the env component.

What it is all about

When using the symfony/website-skeleton, you're using the Dotenv component to configure your application. If this is the case, we can basically skip the database setup via the form in the installation bundle, as the dumped parameters.yml file will not be included anyway.

What it does

• Check for the environment variable DATABASE_URL, if given, assume the application is configured using the env component.
• Show a special error message if using the env component but the connection to the database failed.
• Check the given connection. If it doesn't work fall back to the default behaviour.

Open points

• [x] Translations. So far, this PR only provides the english translation of the two new string misconfigured_database_url and misconfigured_database_url_explain.
• [x] Tests

I probably missed quite a few things, so this is RFC.

[leofeyer]

Looks good to me. 👍

[leofeyer]

Thank you @sheeep.