Add transition to media modal background (#15843)

* Add transition to media modal background

* use reduceMotion

* Move background color transition into css

Signed-off-by: marcin mikołajczak <me@mkljczk.pl>

app/javascript/styles/mastodon/components.scss

@@ -4472,6 +4472,7 @@ a.status-card.compact:hover {
   right: 0;
   bottom: 0;
   background: rgba($base-overlay-background, 0.7);
+  transition: background 0.5s;
 }
 
 .modal-root__container {