BS3: Simpler row margins

This makes negative margins on rows the default, not just in containers, and adds padding to body to compensate for the negative margin on narrow viewports. **Why do this?**

* Consistent row behavior across all viewports and grid tiers
* Consistent row behavior within a variety of components that *aren't* containers
* Simpler styling for completely fluid layouts (those that go edge to edge)

This would fix #9565, #9605, #9664, and a few other issues I believe.