Extras isn't pulling in screenshots

This can be verified by someone with admin access, but I think the issue is that the application_screenshots view is 404'ing:

https://apps.ubuntu.com/cat/applications/screenshots/unity-lens-askubuntu/

because the package isn't marked as the latest package. I get the same 404 locally for the firefox package, and noticed this:

{{{
235 def application_screenshots(request, package_name):
236 import pdb;pdb.set_trace()
237 -> app = get_object_or_404(Application, package_name=package_name,
238 is_latest=True)
239 screenshots = WebServices().get_screenshots_for_package(package_name)
240 return HttpResponse(json.dumps(screenshots),
241 mimetype='application/json')
242
(Pdb) Application.objects.get(package_name=package_name)
<Application: Safe and easy web browser from Mozilla (firefox)>
(Pdb) Application.objects.get(package_name=package_name, is_latest=True)
*** DoesNotExist: Application matching query does not exist.
}}}

The attached branch simply ensures that the screenshot view is not dependent on the redundant (and perhaps-not-set) is_latest field.

Another part of the fix for this should be ensuring that the check_all_latest command is run after each import (import_app_install_data and import_for_purchase), but I want to check with achuni about that first as it looks like the kind of command we wouldn't want to run unnecessarily (ie. after each import of a distroseries, but rather after importing all distroseries - which may be why it's been left manual).

So the attached branch fixed one issue, we'll no longer get a 404 at

https://apps.ubuntu.com/cat/applications/screenshots/unity-lens-askubuntu/

but an empty list. achuni noted:

14:46 < achuni> noodles: hmmm I was going to say that extras uses a custom field on the package data
                to specify the screenshot url, but I can't remember what it was now
14:47 < noodles> achuni: ah k, I'll take a look now.
14:47 < achuni> noodles: we're currently checking the desktop file for the screenshot url, but
                extras doesn't have an app-install-data package
14:47 < achuni> noodles: so that's in the apt-cache data for extras