garrysmod/html/template/addon_list.html
After line 42, add this: <li><a ng-click="OpenAddonManager()">Open Addon Manager</a></li>
So it looks like this:
garrysmod/html/js/menu/control.Menu.js
After line 138, add this: $scope.OpenAddonManager = function() { lua.Run( "RunConsoleCommand( 'addons' )" ); }
So it looks like this: