Performance improvements

https://github.com/pfraze/phoenix/issues/141

Moved most state that is exclusive to rendering the feed into a `feedView` struct. This made it easier to create thunks (called "partials" in Mercury's api) which compare correctly and do not cause frivolous re-renders.
Moving the feed filters *inside* the partials was important. By passing a filtered array into the partial, we were creating a new object on every pass, and so the thunk was guaranteed to fail comparison every time.