Having a `_mixins.scss` file is like what php developers used to do, dropping everything in a `functions.php` file. If we want to be modular I believe a component/module should contain its own mixins. It also means that taking the file `_grid-system.scss` out of the project is possible to make it reusable (for easy prototyping).

We can talk about it if you feel strongly that our architecture should have a clean separation between files with mixins and files that outputs code.