Add support for spoilers on images (MSC4193) (#2212)

* Add support for MSC4193: Spoilers on Media

* Clarify variable names and wording

* Restore list atom

* Improve spoilered image UX with autoload off

* Use `aria-pressed` to indicate attachment spoiler state

* Improve spoiler button tooltip wording, keep reveal button from conflicting with load errors

src/app/components/message/content/style.css.ts

@@ -30,3 +30,10 @@ export const AbsoluteFooter = style([
     right: config.space.S100,
   },
 ]);
+
+export const Blur = style([
+  DefaultReset,
+  {
+    filter: 'blur(44px)',
+  },
+]);