Cast "isFeatured" to "featured"

The appstore returns a "isFeatured" in the current API revision. We need to cast this thus.

Fixes the following error log message when opening the tab "Customization":

> Undefined index: featured at /media/psf/stable9/settings/Controller/AppSettingsController.php#233

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>