Refactor work package index styling (flexible box based solution)

This PR has a broad scope and also includes:
* some refactoring of legacy styles (`main.css`)
* commit from #1605

https://www.openproject.org/work_packages/14811
https://www.openproject.org/work_packages/13498
https://www.openproject.org/work_packages/13597

as collateral:
https://www.openproject.org/work_packages/11935
@ulferts those media query rules were moved to `work_packages.sass` (line 76).