Explicitly declare disabled button css opacity.

Previously this was happening in LMS through edx-platform CSS, but it was not happening in Studio.

openassessment/xblock/static/sass/xb/elements/_controls.scss

@@ -101,6 +101,7 @@
     color: $action-primary-color-disabled;
     border-color: $action-primary-color-disabled;
     background: $action-primary-color-disabled-back;
+    opacity: 1;
   }
 }
 
@@ -134,5 +135,6 @@
     background: $white;
     border-color: $action-primary-color-disabled;
     color: $action-primary-color-disabled;
+    opacity: 1;
   }
 }

openassessment/xblock/static/sass/xb/utilities/_reset.scss

@@ -288,6 +288,14 @@
   }
 
   /**
+   * Set opacity on buttons to 50% (which matches LMS styling).
+   */
+
+  button[disabled] {
+    opacity: 0.5;
+  }
+
+  /**
    * Remove inner padding and border in Firefox 4+.
    */