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/qml/Menu/Timeline/MessageDelegateContextMenu.qml

@@ -226,7 +226,7 @@ Loader {
                         spacing: Kirigami.Units.largeSpacing
                         Kirigami.Avatar {
                             id: avatar
-                            source: author.avatarMediaId ? ("image://mxc/" + author.avatarMediaId) : ""
+                            source: author.avatarSource
                             Layout.preferredWidth: Kirigami.Units.gridUnit * 3
                             Layout.preferredHeight: Kirigami.Units.gridUnit * 3
                             Layout.alignment: Qt.AlignTop