LMS: revising .is-hidden utility class to be used at a global level

lms/static/sass/base/_base.scss

@@ -300,6 +300,11 @@ mark {
   @extend %ui-disabled;
 }
 
+// UI - is hidden
+.is-hidden {
+  display: none;
+}
+
 // UI - semantically hide text
 .sr {
   @extend %text-sr;

lms/static/sass/course/instructor/_instructor_2.scss

@@ -211,11 +211,6 @@ section.instructor-dashboard-content-2 {
     display: block;
     border-radius: 1px;
     padding: ($baseline*0.75) $baseline;
-
-    // CASE: is hidden
-    &.is-hidden {
-      display: none;
-    }
   }
 
   .message-title {

lms/static/sass/discussion/_mixins.scss

@@ -136,10 +136,6 @@
   &:last-child {
     margin-right: 0;
   }
-
-  &.is-hidden {
-    display: none;
-  }
 }
 
 @mixin forum-user-label($color) {

lms/static/sass/discussion/elements/_actions.scss

@@ -12,12 +12,7 @@
 
     .actions-item {
       @include box-sizing(border-box);
-      display: block;
       margin: ($baseline/4) 0;
-
-      &.is-hidden {
-        display: none;
-      }
     }
 
     .more-wrapper {
@@ -82,12 +77,7 @@
     }
 
     .actions-item {
-      display: block;
       margin: 0;
-
-      &.is-hidden {
-        display: none;
-      }
     }
   }
 

lms/static/sass/elements/_system-feedback.scss

@@ -11,10 +11,6 @@
   background: $notify-banner-bg-1;
   padding: $baseline ($baseline*1.5);
 
-  &.is-hidden {
-    display: none;
-  }
-
   // basic object
   .msg {
     @include clearfix();