User directory configuration page should be marked as customizable / configurable

The page XWiki.UserDirectoryConfiguration is not marked in the application as a "content" page in any way, neither as configurable nor as customizable.

I don't know exactly what is the strategy for such pages (pages storing configurations) for the standard distribution, but I noticed that the Dashboard home was marked as customizable by XWIKI-15222, so this issue is about applying the same logic to this page.