msglist: Fix timestamp location

The values I had were copied from web styles, but didn't account for
the padding I've put around the whole thing.

Author: gnprice

lib/message_list.dart

@@ -259,7 +259,7 @@ class MessageWithSender extends StatelessWidget {
               ])),
           Container(
               width: 80,
-              padding: const EdgeInsets.only(top: 8, right: 10),
+              padding: const EdgeInsets.only(top: 4, right: 2),
               alignment: Alignment.topRight,
               child: Text(time, style: _kMessageTimestampStyle))
         ]));