Widget single-column layout styling

@donnapep 
The changes I have made establish a:
- widget class to contain all relevant classes
- .settings and .settings-center classes for horizontally and vertically centering and sets a fixed width of 500px for all breakpoints except mobile (xs, 100%)
- wrapper height % can be overridden in widget and still maintain vertical centering., this will allow a smaller height to be set within the widget
- .section class for styling the new grouping of sections within widget settings

I have kept the .widget-overlay and .widget-wrapper classes for legacy purposes.

This needs reviewing and approval prior to being able to do a PR for Spreadsheet settings as it relies on this being deployed to rise-common-test. Thanks.