enhance FeedItem for feed builder stuff

[ngokevin]

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.

[chuckharmston]

No 400s?

[chuckharmston]

r+wc, add some error handling and