Add support for banner timeout

lib/ui/widgets/banner_widget.dart

@@ -1,5 +1,8 @@
+import 'dart:async';
+
 import 'package:comunic/helpers/server_config_helper.dart';
 import 'package:comunic/models/server_config.dart';
+import 'package:comunic/utils/date_utils.dart';
 import 'package:comunic/utils/intl_utils.dart';
 import 'package:flutter/material.dart';
 import 'package:url_launcher/url_launcher.dart';
@@ -14,17 +17,37 @@ class BannerWidget extends StatefulWidget {
 }
 
 class _BannerWidgetState extends State<BannerWidget> {
+  Timer _timer;
+
+  bool get _shouldShowBanner => showBanner && !_bannerDismissed;
+
   void _hideBanner() {
-    setState(() => _bannerDismissed = true);
+    if (this.mounted) setState(() => _bannerDismissed = true);
   }
 
   void _openLink() {
     launch(srvConfig.banner.link);
   }
 
+  @override
+  void initState() {
+    super.initState();
+
+    if (_shouldShowBanner && srvConfig.banner.expire != null) {
+      _timer = Timer(
+          Duration(seconds: srvConfig.banner.expire - time()), _hideBanner);
+    }
+  }
+
+  @override
+  void dispose() {
+    if (_timer != null) _timer.cancel();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
-    if (!showBanner || _bannerDismissed) return Container();
+    if (!_shouldShowBanner) return Container();
 
     final banner = srvConfig.banner;
     return Card(