design: Make load more button an elevated button

lib/pages/views/chat_view.dart

@@ -277,18 +277,12 @@ class ChatView extends StatelessWidget {
                                             child: CircularProgressIndicator(),
                                           )
                                         : controller.canLoadMore
-                                            ? TextButton(
+                                            ? Center(
+                                              child: OutlinedButton(
                                                   onPressed:
                                                       controller.requestHistory,
                                                   child: Text(
-                                                  L10n.of(context).loadMore,
+                                                      L10n.of(context).loadMore),
-                                                  style: TextStyle(
-                                                    color: Theme.of(context)
-                                                        .primaryColor,
-                                                    fontWeight: FontWeight.bold,
-                                                    decoration: TextDecoration
-                                                        .underline,
-                                                  ),
                                                 ),
                                             )
                                             : Container()