Move common scss mixins to common/static/sass, and tweak the video player to have more in common between the cms and the lms

lms/static/sass/base/_mixins.scss

@@ -1,24 +0,0 @@
-@function em($pxval, $base: 16) {
-  @return #{$pxval / $base}em;
-}
-
-// Line-height
-@function lh($amount: 1) {
-  @return $body-line-height * $amount;
-}
-
-@mixin hide-text(){
-  text-indent: -9999px;
-  overflow: hidden;
-  display: block;
-}
-
-@mixin vertically-and-horizontally-centered ( $height, $width ) {
-  left: 50%;
-  margin-left: -$width / 2;
-  //margin-top: -$height / 2;
-  min-height: $height;
-  min-width: $width;
-  position: absolute;
-  top: 150px;
-}

lms/static/sass/base/_mixins.scss

@@ -0,0 +1 @@
+../../../../common/static/sass/_mixins.scss