autocomplete: Put best matches near input field.

This commit reverses the list that was originally
presented to the user while showing the typeahead
menu.

This makes sense since on mobile its easier to click
on options closer to the input box, i.e. where your
fingers are currently present, instead of pressing
arrow keys on a keyboard which is true on a desktop
setup.

Hence we place the best matching options not at the
top of the typeahead menu, but instead put them at
the bottom for better reachability and convenience of
the user.

Fixes  zulip#1123.
Fixes zulip#1121.

lib/widgets/autocomplete.dart

@@ -134,6 +134,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
               constraints: const BoxConstraints(maxHeight: 300), // TODO not hard-coded
               child: ListView.builder(
                 padding: EdgeInsets.zero,
+                reverse: true,
                 shrinkWrap: true,
                 itemCount: _resultsToDisplay.length,
                 itemBuilder: _buildItem))));