Enable debian packaging with https://packager.io

This pull request adds support for packaging OpenProject for Debian 7 using Packager.io, and automatically setting up OpenProject with a wizard. It is not really meant to be merged as is, but to track the progress of the packaging process on the dev branch, and allow people to make comments.

I will extract a few separate pull requests from this branch as soon as I get back from vacation, to make the review and merging process easier.
You can configure OpenProject just with environment variables. (without doing the erb-parsing). See: https://github.com/opf/openproject/blob/c9359dc31f224c3f3dffd0e55739f38ff497a9c7/doc/CONFIGURATION.md#environment-variables