slimmed down feed element serializers for transonic listing/search

pretty much don't serialize and include every app in feed element responses if we just want to list/search feed elements in curation tools.

a good start for slimming down feeditem views since we don't need every app fully serialized on the feed view either.
If you inherit from `FeedBrandSerializer.Meta` you won't need to define `model` or `url_basename`. Doesn't save many characters, but for OOPurity we should probably do this. /g