Another set of UI updates (FAB returns, a few others) (#281)

* Bring back the FAB and move compose out of the footer

* Increase the touch target sizes of the header back btns (close #279)

* Trigger 'load more' sooner (close #280)

src/view/com/posts/Feed.tsx

@@ -168,7 +168,7 @@ export const Feed = observer(function Feed({
           onScroll={onScroll}
           onRefresh={onRefresh}
           onEndReached={onEndReached}
-          onEndReachedThreshold={0.25}
+          onEndReachedThreshold={0.6}
           removeClippedSubviews={true}
           contentInset={{top: headerOffset}}
           contentOffset={{x: 0, y: headerOffset * -1}}