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/sass/layout/_headers.scss

@@ -7,7 +7,7 @@
     line-height: 56px;
     position: relative;
     width: 100%;
-    z-index: 5;
+    z-index: 9;
 
     .member-popover__trigger,
     .pinned-posts-button {