prevent recursion of version/webapp calling update_status on each other (bug 960044)

http://sentry.dmz.phx1.mozilla.com/addons/marketplace-dev/group/17692/

**Trace:**

- app.update(status=amo.STATUS_PENDING)
- The status update triggers Webapp.watch_status which calls update to update the version’s nomination.
- The nomination update triggers Version.update_status which updates app status by calling Webapp.update_status.
- Webapp.update_status calls app.update(status=amo.STATUS_PENDING)

**Solution:**

In AMO, the ```Version.models.update_status``` post_save signal calls ```Addon.model.update_status```. On Marketplace, we override ```update_status``` in Webapp and call it manually in developer views. Having Version call update_status on Addon/Webapp is an AMO behavior, so this patch checks when ```Webapp.update_status``` is being called from Version and tries not to call ```Webapp.update(status=...)```.