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/LocationDelegate.qml

@@ -71,7 +71,7 @@ TimelineContainer {
                         width: height
                         height: parent.height / 3 + 1
                         name: model.author.name ?? model.author.displayName
-                        source: model.author.avatarMediaId ? ("image://mxc/" + model.author.avatarMediaId) : ""
+                        source: model.author.avatarSource
                         color: model.author.color
                     }
                 }