Compute asynchronously the list of paid extensions that are missing a license

[mflorea]

The list of paid extensions that are missing a license, displayed in the notification panel, is ATM computed synchronously, slowing down the page load. The code is in LicensesNotificationsUIX and what takes the most time is most probably the filtering done in getVisiblePaidExtensions. We cannot drop this filtering (we could move the code to Java to speed it up a bit) but we can modify LicensesNotificationsUIX to load the list of paid extensions using a separate HTTP request, after the main page is loaded, as seeing the notification message is not critical.