Commit Twentyseventeen companion style sheets

Commit is first pass to add the scss,css files to core.

This commit updates the stylesheet to match to 2017 breakpoints & general styles; Corrects any overly apparent style conflicts; updates our mixins and vars where necessary, & is a fairly rounded pass to improve the look of BP run under 2017.

Subsequent passes will address any further style issues encountered & address some visual styling aspects such as backgrounds, borders that are better removed.

See #7338