Improve calculation of the grid recalculation in portlet-grid.less

This is not an issue, just an idea for future possible improvements for portlet-grid.less mixin
https://github.com/Jasig/FeedbackPortlet/blob/master/src/main/webapp/less/portlet-grid.less
MDN has publish a reference on the Grid topic.
This reference can be also useful with any third part grid system (Bootstrap 3, bootstrap 4, others).
https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Grids
See also for the css grid propertie:
http://codepen.io/collection/DgwjNL/