Fix `DisabledComposerView` description being overriden by powerlevel changes (#58)

We added the description in https://github.com/matrix-org/matrix-public-archive/pull/54 but then made a fix to one of the after render errors, https://github.com/matrix-org/matrix-public-archive/pull/57, which exposed that the description is then overriden when the powerlevel changes.

We now just override the getters directly instead of the internal fields which are overwritten internally.

---

This also fixes a warning from Hydrogen in the console:

```
disposable not found, did it leak? {kind: 'disabled', description: Array(3)}
```

shared/4-hydrogen-vm-render-script.js

@@ -262,11 +262,16 @@ async function mountHydrogen() {
     this.navigation.applyPath(path);
   };
 
-  // FIXME: We shouldn't have to dive into the internal fields to make this work
+  Object.defineProperty(roomViewModel, 'timelineViewModel', {
-  roomViewModel._timelineVM = timelineViewModel;
+    get() {
+      return timelineViewModel;
+    },
+  });
   const fromDate = new Date(fromTimestamp);
   const dateString = fromDate.toISOString().split('T')[0];
-  roomViewModel._composerVM = {
+  Object.defineProperty(roomViewModel, 'composerViewModel', {
+    get() {
+      return {
         kind: 'disabled',
         description: [
           `You're viewing an archive of events from ${dateString}. Use a `,
@@ -281,6 +286,8 @@ async function mountHydrogen() {
           ` to start chatting in this room.`,
         ],
       };
+    },
+  });
 
   const archiveViewModel = new ArchiveViewModel({
     // Hydrogen options