FIX: rich editor spoiler on firefox (#36378)

Blurred spoiler's default styles set `user-select: none`, which makes
Firefox treat it as non-editable of sorts (can't click on it to select,
can't navigate it with the keyboard).

This reverts this style when inside the ProseMirror editor.

plugins/spoiler-alert/assets/stylesheets/discourse_spoiler_alert.scss

@@ -61,4 +61,8 @@
     box-shadow: 0 0 4px 4px rgb(var(--primary-rgb), 0.2);
     max-width: 100%;
   }
+
+  &.spoiler-blurred {
+    user-select: auto;
+  }
 }