Any registered user can edit the default Flamingo theme

This issue happens because the default Flamingo theme is located inside the FlamingoThemes space, which doesn't have any rights set. In previous XWiki versions there was only the ColorThemes space that included all themes and which had the right to Edit allowed only for Admins.

We need to set the right also for the FlamingoThemes space so we don't end up with ModifiedTheme.png.