Fixes #10903 and #10457: IE8-9 don't support events on elements without background-color, so we have to hack it

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

The left control should be positioned from the left, while the right
control should be positioning from the right. Positioning both from the
left results in the right carousel control being incorrectly positioned
(most noticeable with a larger than normal font size).

Moved alerts, badges, button groups, buttons, carousel, close to
variables.less and customize.html

* Remove `@carousel-control-hover-color` since that's just the same as
the default control color and all we change is `opacity` (it's required
for specificity).
* Add vars to customizer
* Update customizer nav to include new section
* Remove the `li` from the var names
* Consolidate and simplify `text-shadow` vars

moved buttons.less, carousel.less & close.less in this commit

* No longer hidden by default--up to users to figure that part out with
longer content
* Cleaned up styles and spacing

Used laquo/raquo for control buttons
Updated docs

* Any mentions of .btn-mini
* Old docs page that somehow found it's way back

* Add start and end support to the horizontal gradient as well
* Change all variables from camelCase to use-dashes
* Better comments in the gradients mixins area
* Update current uses of gradients in dropdowns and carousel to specify only two values by direct assignment of variables

refactor some of the carousel caption stuff to center it and tighten styles up; update carousel template to start to redo mobile first

* They're hidden by default in the mobile-first views because otherwise they just cover up the slides
* In mobile they're centered, and then are shown left aligned at larger viewports

* Drop custom type arrow controls for glyphicons icons instead for more consistent rendering
* Remove the captions and instead let folks implement what they want in the carousel
* Move the indicators to the bottom middle instead of top right
* Lighten up gradients on the edges for less harsh lighting

lessified .carousel-caption

Mostly doubling-up :hover styles to also cover :focus, as a first step
to making the framework more keyboard-friendly.
Additionally, fixed two small markup issues in the docs/examples to
make the "Learn more" large primary button-styled links
keyboard-focusable (as without href they're treated as non-tabable
anchors).