FIX: Give the topic timeline unique keys for state

We've observed an error where the back button is displayed improperly in
the topic timeline. It's unfortunately been hard to reproduce but we
suspect it's related to leftover state when re-rendering.

This fix optimistically tries to fix the error by introducing the
topic's id to the unique key the widgets use for state. We can deploy
this and keep an eye out for the bug in the future.

app/assets/javascripts/discourse/app/widgets/topic-timeline.js

@@ -144,7 +144,7 @@ createWidget("timeline-padding", {
 
 createWidget("timeline-scrollarea", {
   tagName: "div.timeline-scrollarea",
-  buildKey: () => `timeline-scrollarea`,
+  buildKey: (attrs) => `timeline-scrollarea-${attrs.topic.id}`,
 
   buildAttributes() {
     return { style: `height: ${scrollareaHeight()}px` };
@@ -411,7 +411,7 @@ createWidget("timeline-footer-controls", {
 export default createWidget("topic-timeline", {
   tagName: "div.topic-timeline",
 
-  buildKey: () => "topic-timeline-area",
+  buildKey: (attrs) => `topic-timeline-area-${attrs.topic.id}`,
 
   defaultState() {
     return { position: null, excerpt: null };