Unify getUser

Always use `NeoChatRoom::getUser` for getting user details in a room context.
- `MessageEventModel` now calls `getUser`
- `getUser` is now overloaded to also be able to take a `NeoChatUser*`
- add params to `getUser` that are needed for the model outputs

src/qml/Component/Timeline/ReplyComponent.qml

@@ -100,7 +100,7 @@ Item {
             implicitHeight: Kirigami.Units.iconSizes.small
 
             source: root.author.avatarSource
-            name: root.author.displayName || ""
+            name: root.author.displayName
             color: root.author.color
         }
         QQC2.Label {