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/Panel/RoomDrawer.qml

@@ -258,7 +258,7 @@ Kirigami.OverlayDrawer {
                             implicitWidth: height
                             sourceSize.height: Kirigami.Units.gridUnit + Kirigami.Units.smallSpacing * 2.5
                             sourceSize.width: Kirigami.Units.gridUnit + Kirigami.Units.smallSpacing * 2.5
-                            source: avatar ? ("image://mxc/" + avatar) : ""
+                            source: avatar
                             name: model.userId
                         }