Incorporate Bootstrap v4

[jdmdigital]

The vision is to create an option in the theme Customizer to use either our expanded version of Bootstrap v3.3.7 or v4 with some fallbacks. For backwards compatibility reasons, we'll set this option to default to the legacy version.

Version 4 of Bootstrap uses flexbox which isn't supported by older browsers. I think it makes sense to allow the developer to make the decision on whether to use this latest version or the older one if compatibility with older browsers is a design requirement.

However, because this would require a significant amount of work, we're opening it up for discussion (over beers, of course).