sticky_header: Rewrite sticky headers completely

In particular this gives us two features we'll want in the Zulip
message list when we go to make recipient headers appear only
where needed (#174):

 * An item's header now gets built (and laid out and painted) only if
   the item is actually the one at the relevant edge of the viewport
   such that the stickiness comes into play.  This lets us attach a
   sticky header to every message, even when it shares a recipient
   header with the message that comes before it.

 * An item can now (optionally) allow the header's stickiness at the
   edge of the viewport to take precedence over the header's
   confinement within the item's own bounds.  We'll use this to let
   the header stay pinned when scrolling between items that share a
   recipient header.

Author: gnprice

lib/widgets/message_list.dart

@@ -382,9 +382,11 @@ class MessageItem extends StatelessWidget {
       shape: Border(
         left: recipientBorder, bottom: restBorder, right: restBorder));
 
+    final recipientHeader = RecipientHeader(message: message);
     return StickyHeaderItem(
-      header: RecipientHeader(message: message),
-      content: Column(children: [
+      header: recipientHeader,
+      child: Column(children: [
+        recipientHeader,
         DecoratedBox(
           decoration: borderDecoration,
           child: MessageWithSender(message: message, content: item.content)),