Conflicts:
	less/_mixins.less

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

FIXES #12994

FIXES #12992

added .transition-timing-function(@timing-function) mixin, defaults to @timing-function.

The col-*-push-0 and col-*-pull-0 classes try to reset the positioning using 0% but this prevents the opposite direction positioning to freeze and not being set correctly.
To fix that, these must set the position to auto instead of 0% with means left:auto and right:auto instead of left:0% and right:0%.

I think that's the only animation property that was missing and it's quite useful.

https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode

No need for old Firefox versions support.

New stuff:

* Adds `.has-feedback` and `.form-control-feedback` to show icon-based
form feedback to users. Fixes #11836.
* Removes the `width: 1em` on `.glyphicon` because it makes it
impractical to resize the width of all icons, even if they’re not
“empty” (e.g., a simple `.glyphicon { width: 30px; }` wouldn’t work,
nor would a class preceeded by a parent class.

Clean up:

* Sets `width: auto;` on all `.form-control`s within inline forms.
* Removes all the inline `select` menu `width` styles because all
inputs are now `auto`.
* Update form states docs to separate out sections better.