* Darken dismiss icon opacity
* Darken text on active list group items
* Darken light gray for accessibility/contrast, then update gray and dark gray to match

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

* Remove dupe hover color var (again, it's not necessary to have it's
own var here since we're controlling it via `opacity`)
* Add to customizer

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

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).

breaking out patterns.less even more, removing unnecessary div from checkbox and radio lists--now just labels and inputs