Closes #13103 by merging it.

Fixes #13066: Add z-index to the form control feedback icons so input groups don't render on top of them

Fixes #13003: Move padding on radio and checkbox options from the surrounding div to the label to remove the no-click deadzone caused by negative margin

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

FIXES #12994

Ensures proper sizing and alignment of input groups within inline and
navbar forms.

* Uses `inline-table` on the input group
* Nukes the widths to `width: auto`—without this, the parent input
group doesn’t size correctly and functions as `display: table;` or
`block`

* Removes default max-height from .navbar-collapse (so not every navbar
will get a max-height collapse section)
* Scopes regular 340px max-height to fixed top and bottom navbars only
(those are the only ones that really need a max-height to enable
scrolling)
* Adds a landscape media query for phones to cap the max-height at
200px for fixed navbars"

Fixes #12843: Scope label styles for horizontal forms to a media query so their narrow viewport display looks just like a normal form

Fixes #12669: Properly reset line-height on date inputs for all sizes; Fix IE8+'s misaslignment of text within date inputs

FIXES #12992