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/Dialog/UserDetailDialog.qml

@@ -19,7 +19,7 @@ Kirigami.OverlaySheet {
     property var user
 
     property string displayName: user.displayName
-    readonly property string avatar: room.avatarForMember(user)
+    readonly property url avatar: room.avatarForMember(user)
 
     parent: applicationWindow().overlay
 
@@ -45,7 +45,7 @@ Kirigami.OverlaySheet {
                 Layout.preferredHeight: Kirigami.Units.iconSizes.huge
 
                 name: displayName
-                source: avatar ?? ""
+                source: avatar
                 color: user.color
             }