Merge branch 'fix-sidebar-exapnd' into 'master'

Fix transparent bg for expand sidebar button
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

For #14838 

cc @jschatz1 

See merge request !3520

app/assets/stylesheets/framework/sidebar.scss

@@ -200,7 +200,7 @@
   top: 0;
   left: 0;
   font-size: 20px;
-  background: transparent;
+  background: #fff;
   height: 59px;
   text-align: center;
   line-height: 59px;