DEP-18: Encourage Continuous Integration and Merge Request based Collaboration for Debian packages

This is nearly a complete rewrite of the first DEP-18 draft and supersedes !8 (closed).

Many commenters expected a DEP to be more technical and less about general principles, so this new version is now more specific. Many also objected to labeling collaborative software development with commits being published in real-time and code reviews happening in the open as "true open source". Several also objected to requiring Debian Developers in general to use Salsa as a platform. These concerns are addressed by narrowing the title and the scope of the proposal to just describe using Salsa for collaboration without promoting more inclusive development in general.

Debian has a strong culture of individual maintainers walking their own paths. This version is less generally normative and allows more room for Debian Developers to adopt it in their own ways and pace.