Fix extensions panel style (#29273)

* Stop empty state scrolling

* Fix separator inset, separator vertical spacing and margin above the add extensions button

* Add screenshot for add extensions button spacing

* Add variable for add extensions overlap

* use data-testid

* Fix snapshots

test/unit-tests/components/structures/__snapshots__/RoomView-test.tsx.snap

@@ -2121,6 +2121,7 @@ exports[`RoomView video rooms should render joined video room view 1`] = `
       >
         <aside
           class="mx_RightPanel"
+          data-testid="right-panel"
           id="mx_RightPanel"
         >
           <div

test/unit-tests/components/views/elements/__snapshots__/AppTile-test.tsx.snap

@@ -4,6 +4,7 @@ exports[`AppTile destroys non-persisted right panel widget on room change 1`] =
 <DocumentFragment>
   <aside
     class="mx_RightPanel"
+    data-testid="right-panel"
     id="mx_RightPanel"
   >
     <div