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/neochatroom.cpp

@@ -1921,7 +1921,12 @@ QByteArray NeoChatRoom::roomAcountDataJson(const QString &eventType)
 QUrl NeoChatRoom::avatarForMember(NeoChatUser *user) const
 {
 #ifdef QUOTIENT_07
-    return connection()->makeMediaUrl(memberAvatarUrl(user->id()));
+    auto avatar = connection()->makeMediaUrl(memberAvatarUrl(user->id()));
+    if (avatar.isValid() && avatar.scheme() == QStringLiteral("mxc")) {
+        return avatar;
+    } else {
+        return QUrl();
+    }
 #else
     QUrl url(QStringLiteral("mxc://%1").arg(user->avatarMediaId()));
     QUrlQuery q(url.query());