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/utils/constants.jsx

@@ -493,7 +493,7 @@ export const Constants = {
             buttonBg: '#166de0',
             buttonColor: '#ffffff',
             errorTextColor: '#fd5960',
-            mentionHighlightBg: '#fff2bb',
+            mentionHighlightBg: '#ffe577',
             mentionHighlightLink: '#166de0',
             codeTheme: 'github',
             image: mattermostThemeImage