Subwikis cannot be configured from main wiki if extension is installed in both.

This is a regression introduced by PIWIK-7

Scenario: Subwkis as "project wikis"; only global users.

In that case one only want to configure the Piwik-seerver only in the main wiki, and not again in the subwikis. As the extension needs to be installed (but not configured) in the subwikis for the UI Extension to work, there is an empty configuration page in every subwiki that should be ignored in this case. Since PIWIK-7 this does no longer happen; one need to set (and update) the configuration in every subwiki separately.