Overhaul navbar alignment (.pull-) utilities (fixes #9440)

Now navbars have their own float utility classes, mixin-ed versions of
`.pull-left` and `.pull-right`.

* Removed chained `.navbar-nav.pull-right`
* Added `.navbar-left` and `.navbar-right` as mixins of the default
float utilities, but only above the `@grid-float-breakpoint` to match
the new navbar behavior
* Updated components docs to match the new changeas
* Added callout about component alignment here to explain the new
classes