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/ChatBox/ReplyPane.qml

@@ -14,7 +14,7 @@ GridLayout {
     id: root
     property string userName
     property color userColor: Kirigami.Theme.highlightColor
-    property var userAvatar: ""
+    property url userAvatar: ""
     property var text
 
     rows: 3