resolves local Sass warnings/errors around missing mixins referenced in xmodule styling

cms/static/sass/_mixins-inherited.scss

+../../../common/static/sass/_mixins-inherited.scss
\ No newline at end of file

common/lib/xmodule/xmodule/css/capa/display.scss

@@ -566,7 +566,7 @@ section.problem {
     }
 
     .save {
-      @extend .blue-button;
+      @extend .blue-button !optional;
     }
 
     .show {

common/lib/xmodule/xmodule/css/combinedopenended/display.scss

@@ -577,7 +577,7 @@ section.open-ended-child {
     margin-top: 20px;
 
     input.save {
-      @extend .blue-button;
+      @extend .blue-button !optional;
     }
 
     .submission_feedback {

common/lib/xmodule/xmodule/css/sequence/display.scss

+$border-color: #C8C8C8;
+
+// repeated extends - needed since LMS styling was referenced
+.block-link {
+  border-left: 1px solid lighten($border-color, 10%);
+  display: block;
+
+  &:hover {
+    background: none;
+  }
+}
+
+.topbar {
+  @include clearfix();
+  border-bottom: 1px solid $border-color;
+
+  @media print {
+    display: none;
+  }
+
+  a {
+    &.block-link {
+      border-left: 1px solid lighten($border-color, 10%);
+      display: block;
+
+      &:hover {
+        background: none;
+      }
+    }
+  }
+}
+
+// ====================
+
 nav.sequence-nav {
   // TODO (cpennington): This doesn't work anymore. XModules aren't able to
   // import from external sources.
@@ -294,7 +328,7 @@ nav.sequence-bottom {
   text-align: center;
 
   ul {
-    @extend .clearfix;
+    @include clearfix();
     @include inline-block();
 
     li {