Add DDEV integration

[martin-helmich]

This PR adds a simple integration for DDEV.

This integration consists of a few simple building blocks:

• The mw ddev render-config command may be used to create a project-/app-specific configuration file
• The mw ddev init command initializes a DDEV project in the local directory, whose configuration matches the
  specified mittwald app
• External to this repository, there is also the mittwald/ddev addon, which adds support for pushing+pulling code from+to a remote project