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/Component/Timeline/AvatarFlow.qml

@@ -21,7 +21,7 @@ Flow {
             implicitHeight: avatarSize
 
             name: modelData.displayName
-            source: modelData.avatarMediaId ? ("image://mxc/" + modelData.avatarMediaId) : ""
+            source: modelData.avatarSource
             color: modelData.color
         }
     }