Update to Bootstrap v4

[Alanaktion]

This can initially be developed with the Bootstrap v4 alphas. We should parse+cache the SCSS code in PHP and allow a user to choose between a light and dark varient of the base BS4 theme. I'd like to move away from the custom CSS files used from Bootswatch for more consistency in metrics. We could also consider adding support for a site-wide configurable brand color that would override brand-primary in _variables.scss.