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/TimelineContainer.qml

@@ -381,7 +381,7 @@ ColumnLayout {
             visible: root.showAuthor &&
                 Config.showAvatarInTimeline &&
                 (Config.compactLayout || !showUserMessageOnRight)
-            name: root.author.name ?? root.author.displayName
+            name: root.author.displayName
             source: root.author.avatarSource
             color: root.author.color