Channel switcher UI improvements (#6564)

* Channel switcher UI improvements

* Reverting mattermost theme

* Fixing spacing above the button

* Updating switcher button

* PLT-6691 - Fixing tablet header

webapp/sass/responsive/_tablet.scss

@@ -10,7 +10,7 @@
 
     .channel-header {
         .heading {
-            max-width: 100px;
+            max-width: 90px;
         }
     }