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).

adding actual images to carousel to ensure you can see things are animating; updated styles of carousel controls to work on light and dark backgrounds and use opacity for effects