feat: make showing user presence info optional

895de76e70 replaced the stories feature
with presence status messages. Stories were an optional feature but
right now presence information can only be disabled on a homeserver
level.

Introduce a setting to make this feature optional on a client level.

lib/config/app_config.dart

@@ -49,6 +49,7 @@ abstract class AppConfig {
   static bool sendTypingNotifications = true;
   static bool sendPublicReadReceipts = true;
   static bool? sendOnEnter;
+  static bool showPresences = true;
   static bool experimentalVoip = false;
   static const bool hideTypingUsernames = false;
   static const bool hideAllStateEvents = false;

lib/config/setting_keys.dart

@@ -28,4 +28,5 @@ abstract class SettingKeys {
       'chat.fluffy.send_public_read_receipts';
   static const String sendOnEnter = 'chat.fluffy.send_on_enter';
   static const String experimentalVoip = 'chat.fluffy.experimental_voip';
+  static const String showPresences = 'chat.fluffy.show_presences';
 }