Improve User Avatar Model Roles

- Update the message event and user models so that the full source url is output.
- Separate the reply author into its own role
- Create an empty user object that can be passed so that the QML code no longer needs to check if certain parameters exist.
- Make avatarForMember return and empty QUrl if a valid avatar cannot be found and make use in the user and event models

As well as cleaning up the QML this should also stop the QML Image: Media id '' doesn't follow server/mediaId pattern spam in the log.

src/models/userlistmodel.cpp

@@ -71,7 +71,8 @@ QVariant UserListModel::data(const QModelIndex &index, int role) const
         return user->id();
     }
     if (role == AvatarRole) {
-        return user->avatarMediaId(m_currentRoom);
+        auto neoChatUser = static_cast<NeoChatUser *>(user);
+        return m_currentRoom->avatarForMember(neoChatUser);
     }
     if (role == ObjectRole) {
         return QVariant::fromValue(user);