Minor UI fixes for 4.0 (#6899)

* PLT-7056 - Fixing profile popover position

* PLT-7082 - Removing extra space below header

* PLT-7074 - Channel header fix

* PLT-7054 - Fixing scroll inside members popover

* Updating mention highlight color for default theme

webapp/sass/components/_popover.scss

@@ -242,6 +242,7 @@
             border-top: 1px solid;
             cursor: pointer;
             height: 45px;
+            overflow: hidden;
             padding: 5px 19px 5px 17px;
 
             &:hover {