msglist: Have content pad bottom inset when the compose box isn't shown

This takes advantage of some of StickyHeaderListView's default
behavior (inherited from ListView). From the doc on ListView:

https://api.flutter.dev/flutter/widgets/ListView-class.html
> By default, ListView will automatically pad the list's scrollable
> extremities to avoid partial obstructions indicated by
> MediaQuery's padding. To avoid this behavior, override with a zero
> padding property.

The code changes here are just to arrange for that default behavior
to help us out: we let the real height of the bottom inset propagate
down to the StickyHeaderListView, with the MediaQuery mechanism.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -61,8 +61,10 @@ class _MessageListPageState extends State<MessageListPage> {
               // MediaQuery.removePadding with `removeTop: true`.
               context: context,
 
-              // The compose box pads the bottom inset.
-              removeBottom: true,
+              // The compose box, when present, pads the bottom inset.
+              // TODO this copies the details of when the compose box is shown;
+              //   if those details get complicated, refactor to avoid copying.
+              removeBottom: widget.narrow != const AllMessagesNarrow(),
 
               child: Expanded(
                 child: MessageList(narrow: widget.narrow))),
@@ -228,6 +230,10 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
         color: Colors.white,
         // Pad the left and right insets, for small devices in landscape.
         child: SafeArea(
+          // Don't let this be the place we pad the bottom inset. When there's
+          // no compose box, we want to let the message-list content pad it.
+          bottom: false,
+
           child: Center(
             child: ConstrainedBox(
               constraints: const BoxConstraints(maxWidth: 760),