msglist: Put edit-message progress bar in top half of 4px bottom padding

This is where the progress bar for outbox messages will go, so this
is for consistency with that. Discussion:
  zulip#1453 (comment)

[chris: fixed to maintain 4px bottom padding in the common case
where the progress bar is absent]

Co-authored-by: Chris Bobbe <[email protected]>

Author: PIG208

lib/widgets/message_list.dart

@@ -1598,7 +1598,7 @@ class _EditMessageStatusRow extends StatelessWidget {
     return switch (status) {
       // TODO parse markdown and show new content as local echo?
       false => Padding(
-        padding: const EdgeInsets.only(bottom: 4),
+        padding: const EdgeInsets.only(bottom: 2),
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.stretch,
           spacing: 1.5,