Merge 3.9 RC5 fixes into master (#6459)

* UI fixes for 3.9 (#6400)

* UI fixes for header icon alignment

* PLT-6569 - Fixing pin icon in header

* PLT-6505 - Updating z-index for emoji picker

* Updating z-index for overlaying sidebar

* PLT-6134 - Fix for emoji picker on comments

* Reverting emoji picker fix

* Fixing alignment for header icons

* Removing channel title changes

* Don't show mention badge for active channel (#6411)

* PLT-6583 - Fixing RHS compact view (#6413)

webapp/components/rhs_comment.jsx

@@ -379,7 +379,7 @@ export default class RhsComment extends React.Component {
         }
 
         if (this.props.compactDisplay) {
-            className += 'post--compact';
+            className += ' post--compact';
         }
 
         if (post.is_pinned) {