enhance FeedItem for feed builder stuff

Need some stuff for the Feed Builder. @chuckharmston 

- add order field to FeedItem
- add feed_type field to FeedItem
- add a function-based DRF view to set feeds by region

Data from feed builder will look like:

    self.data = {
            'us': [
                ['app', self.feed_apps[1].id],
                ['collection', 12L],
                ['brand', self.brand.id],
                ['app', self.feed_apps[0].id],
            ],
            'cn': [
                ['brand', self.brand.id],
                ['app', self.feed_apps[2].id],
                ['collection', 12L],
                ['collection', 14L],
            ]
        }

We delete all FeedItems for regions passed in, then all FeedItems passed in will be in one bulk_create.
Few more tests?

- 404 for nonexistent regions
- 400 when you try to add nonexistent feed items