Merge branch 'v4-dev' of https://github.com/twbs/bootstrap into patch-1

diff --cc scss/_custom-forms.scss
index ebca1e1ec24db5a1c80da2773f06e2a62f1c9503,16527eb262920f093c3e75215d8a4dd7befd0b88..ae5fc3d9345dfe75b76418caa4c35b761d3a6ace
--- 1/scss/_custom-forms.scss
--- 2/scss/_custom-forms.scss
+++ b/scss/_custom-forms.scss
@@@ -135,14 -135,10 +135,14 @@@
    padding: .375rem 1.75rem .375rem .75rem;
    padding-right: .75rem \9;
    vertical-align: middle;
 +<<<<<<< HEAD
 +      background: #fff url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCA0IDUiPjxwYXRoIGZpbGw9IiMzMzMiIGQ9Ik0yIDBMMCAyaDR6bTAgNUwwIDNoNHoiLz48L3N2Zz4=) no-repeat right .75rem center;
 +=======
    background: #fff url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAUCAMAAACzvE1FAAAADFBMVEUzMzMzMzMzMzMzMzMKAG/3AAAAA3RSTlMAf4C/aSLHAAAAPElEQVR42q3NMQ4AIAgEQTn//2cLdRKppSGzBYwzVXvznNWs8C58CiussPJj8h6NwgorrKRdTvuV9v16Afn0AYFOB7aYAAAAAElFTkSuQmCC) no-repeat right .75rem center;
    background-image: none \9;
 +>>>>>>> twbs/v4-dev
    background-size: 8px 10px;
-   border: 1px solid $input-border;
+   border: $input-border-width solid $input-border-color;
    // Use vendor prefixes as `appearance` isn't part of the CSS spec.
    -moz-appearance: none;
    -webkit-appearance: none;