Break mixins.less up into smaller modules.

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994
Merge branch 'master' into modular-the-mixins

* master:
  Replace some images on the Carousel example with some base64 encoded 1x1 gifs
  ugh grunt
  moar grunt
  mo copy
  http://www.youtube.com/watch?v=AJWtLf4-WWs
  grunt
  Copy change
  fix grammar in comment in forms.less
  Fixes #12759
  Fixes #12738
  replace .visible-{size} with .visible-{size}-{display} ; fixes #8869

Conflicts:
	dist/css/bootstrap-theme.css.map
	dist/css/bootstrap.css.map
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap-theme.css.map
	docs/dist/css/bootstrap.css.map
	less/mixins.less
Add consistent newline at end of files in less/mixins
Add recompiled assets after nuking toc in mixins.less.
Formatting, comment consolidation, and line endings
Merge branch 'master' into modular-the-mixins

Conflicts:
	dist/css/bootstrap.css.map
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap.css.map
words
Merge branch 'master' into modular-the-mixins
even more grunt
Merge pull request #12998 from twbs/modular-the-mixins

Modular the mixins