Websites might use different weights such as medium or thin or use light as default weight instead of normal.
Also referenced: #18605 and #16170.

Fix comment style to `//`, because other than the initial attribution comment, [we don't want any comments in the compiled CSS](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#comments).

Reverted to original content from https://raw.githubusercontent.com/necolas/normalize.css/d311749e127ea3ea13eb900ad953270d62734f4f/normalize.css

https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850395
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850397
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850398
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850401
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850399
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850402
https://github.com/twbs/bootstrap/pull/19639#discussion-diff-57850403

#19575

Bump Normalize to https://github.com/necolas/normalize.css/commit/fe567632afcf8afcf17925ab2a507ed87504cce8

So as to pull in https://github.com/necolas/normalize.css/pull/496