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/StateDelegate.qml

@@ -82,7 +82,7 @@ QQC2.Control {
                         implicitHeight: Kirigami.Units.iconSizes.small
 
                         name: modelData.displayName
-                        source: modelData.avatarMediaId ? ("image://mxc/" + modelData.avatarMediaId) : ""
+                        source: modelData.avatarSource
                         color: modelData.color
                     }
                 }
@@ -121,7 +121,7 @@ QQC2.Control {
                 visible: !columnLayout.folded || stateEventRepeater.count === 1
 
                 name: modelData.author.displayName
-                avatar: modelData.author.avatarMediaId ? ("image://mxc/" + modelData.author.avatarMediaId) : ""
+                avatar: modelData.author.avatarSource
                 color: modelData.author.color
                 text: `<style>a {text-decoration: none;}</style><a href="https://matrix.to/#/${modelData.author.id}" style="color: ${modelData.author.color}">${modelData.authorDisplayName}</a> ${modelData.text}`