Show the time when we were invited on the invite page

Currently, this is something that's missing even in clients like Element
(!?!) I will sometimes miss when invites are sent - like when I'm
sleeping - and then when I start back up NeoChat I have no idea when
they tried to invite me. This can get worse on bigger timescales, like
when you go on vacation for a few days.

src/libneochat/neochatroom.cpp

@@ -124,6 +124,7 @@ NeoChatRoom::NeoChatRoom(Connection *connection, QString roomId, JoinState joinS
             updatePushNotificationState(u"m.push_rules"_s);
 
             Q_EMIT canEncryptRoomChanged();
+            Q_EMIT inviteTimestampChanged();
         },
         Qt::SingleShotConnection);
     connect(this, &Room::changed, this, [this] {
@@ -1665,6 +1666,15 @@ QString NeoChatRoom::invitingUserId() const
     return event->senderId();
 }
 
+QDateTime NeoChatRoom::inviteTimestamp() const
+{
+    auto event = currentState().get<RoomMemberEvent>(connection()->userId());
+    if (!event) {
+        return {};
+    }
+    return event->originTimestamp();
+}
+
 void NeoChatRoom::setRoomState(const QString &type, const QString &stateKey, const QByteArray &content)
 {
     setState(type, stateKey, QJsonDocument::fromJson(content).object());