mediawiki.ui: Introduce grid system
A set of mixins for the creation of responsive layouts:
- Variables for the screen sizes to support (can be extended).
- Mixins to indicate the width and visibility of elements per breakpoint (or in general).
- This also enables the specification of any CSS property for each breakpoint.
This is based on the RfC: https://www.mediawiki.org/wiki/Requests_for_comment/Grid_system
More information at: https://pauginer.github.io/agora-grid/