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.
I don't really like the `-mini` pattern. This should probably be part of content negotiation, figure out which serializer to use through a header.