Fix for "Find your extensions" feature

The feature was broken a while ago but I haven't had the time to investigate. Fortunately, the fix only required an additional parameter (i.e. `category=extensions`) to the XHR as it appeared to default to Apps otherwise, showing no results for extensions developers.

Perhaps one day we can easily support both apps and extensions, but this would require double requests in this case (and maybe others). This fix is the easiest for now.