Flagged upstream in bootstrap-sass by scss-lint

Refactor alerts to have a base class, but no default class, thus requiring a modifier class at all times

fixes #9310: alerts aren't dismissable by default, now requires .alert-dismissable if you need a close button

Added padding variable for alerts and changed customize.html Warning state (which doesn't exist anymore)

Resolves https://github.com/twbs/bootstrap/issues/9014. updates mixins to consistently use semicolons as argument separator. Rebuild to confirm, no diff in compiled CSS.

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

* No more children selectors to set all links in alerts as a special color because buttons and more wrongly inherited color
* Adds .alert-link for easy styling in any alert

* 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

remove .border-radius mixin and replace with standard border-radius property, since -webkit-* and -moz-* are only needed for older versions