add dialog when first seeing banned user list

Fixes #113

lib/controllers/banned_warning_dismissed.dart

@@ -0,0 +1,20 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/shared_prefs.dart";
+
+class BannedWarningDismissedController extends AsyncNotifier<bool> {
+  static const String key = "bannedWarningDismissed";
+
+  @override
+  Future<bool> build() async =>
+      await ref.watch(SharedPrefsController.provider).getBool(key) ?? false;
+
+  Future<void> set(bool value) async {
+    state = .data(value);
+    await ref.read(SharedPrefsController.provider).setBool(key, value);
+  }
+
+  static final provider =
+      AsyncNotifierProvider<BannedWarningDismissedController, bool>(
+        BannedWarningDismissedController.new,
+      );
+}