Turns out we were nixing the default browser spacing for bullets and
numbers, so this reverts that and instead just twerks the top/bottom
`margin`.

* New padding approach with separate horizontal and vertical padding
variables
* Improved sizing in large and small buttons and form controls
* Dropped the `.btn-mini` (since we have no `.input-mini` to match, and
holy fuck those were small buttons)
* Dropped the `.pagination-mini` as well because once again, to hell
with such small components
* Changed `@line-height-headings` to `@headings-line-height`
* Removed the `@headings-font-family` because it was honestly kind of
useless

* Instead of 20px as @line-height-base, use 1.5
* Update typographic scale for headings to use unitless as well--required some twerking of decimals a bit and some rounding
* Introduce new @line-height-computed value, which takes base font-size and multiplies it by 1.5, for use in padding and margin on components

* Instead of shared and separate CSS, isolate by class for easier mixin-ability
* .list-unstyled is now used as a mixin in .list-inline

* Added a new mixin to calculate rems and pixels for IE fallback
* Updated the body, p, and .lead elements to use the new .font-size() mixin
* New mixin has default value of 14px or 1.4rem
* Still needs exploration, and change this means lots of work rejiggering margins and line-heights
* Related, started to reorganize the mixins a bit to call out mixins that are used by the framework and not utilities or snippets

This relates to changes in issue #7284

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).