Fix UserFeed loader

The UserFeed loading spinner still incorrectly displays in some circumstances. In particular it fails when posts have been made and then are all deleted. The only case the spinner should display is where both all sources have not yet been attempted, and there is nothing to display. Currently a callback exists that fires when all sources have been attempted, but the feed has no tombstones. This switches to just always fire after all sources have been attempted if they are tombstones or not, and updates the loader to only display when not all the sources are attempted and the feed is empty.