Make extensions tab redirect if no extensions are installed

Currently, when no extensions are installed, the Extensions tab does not show up. This causes confusion for some people, as the Extensions tab does show up in the ACP module management.

As an alternative, the Extensions tab could be always displayed. Additionally, if there are not Extensions installed, one would want to redirect the user to the Customise page if he/she presses the Extensions tab.
This would actually automate the much-given advice 'You should first install a extension on the customasation page, then the extensions tab will be visible.'

Taken from this post:
https://www.phpbb.com/community/viewtopic.php?p=13819866#p13819866