fixes #10115: Default carousel controls and Glyphicon controls should behave the same on small devices and up

Hadn't thought about this until fixes #10137, but it makes sense I
think to have margin on the bottom of thumbnails for spacing when
stacking (or using different grid tiers).

* Update styling of justified nav links to be more mobile friendly
(with all four rounded corners and some margin between items)
* Add note to docs to callout bogus WebKit rendering on component and
example

Added th selector in .table-row-variant mixin

iOS doesn't pick up the z-indexing properly on the dropdowns unless you
specify one for the parent navbar.

Refactor panels to have a base class for improved flexibility when customizing (and no overriding to start, too)

* Extract color styles from .navbar { } and put them into
.navbar-default { }
* Change variable names
* Change the docs accordingly (i.e., add the .navbar-default class to
all default navbars)
* Change customizer accordingly

fixes #9790: nullify right padding in container-ed navbars when last element in collapse is a right aligned navbar nav