Catch app leak during convert

[radiac]

In several scenarios I've seen an app instance reference be carried across during a nanodjango convert operation. This really isn't desirable - nanodjango shouldn't be anywhere near a full Django project, so it's a sign that nanodjango's converter didn't understand what was happening.

The conversion process is complicated so this sort of thing is to be expected, but it's going to cause some weird and wonderful errors.

Detect when Django is being instantiated after a convert, and show a warning that something looks wrong and will need to be fixed manually.