Actor Feed Loaders

[EricBAndrews]

Frontend changes for the real PR.

Contains two minor changes of note:

• EndOfFeedView can now take in an optional loadMore callback; if available, a "Load More" button will be rendered at the bottom of the feed if the loadingState is idle. Sometimes hiding read posts creates a feed that won't trigger the infinite load; this handles that case without needing to do weird threshold hacks. This will also make implementing Disable Infinite Scrolling Option #832 trivial.
• loadFeed now triggers if the loader is empty regardless of loadingState. The middleware PR changes the initial loadingState to be .loading, which in turn results in a spinner being immediately rendered when an empty feed is opened.