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.
Can we docstring all these? Even if it's repetitive, it's nice to be able to use `FeedAppSearchSerializer.__doc__` in the REPL.