Fix RTL layout not being RTL within the columns area (mastodon#10990)

Author: Gargron

app/javascript/styles/mastodon/rtl.scss

@@ -43,6 +43,10 @@ body.rtl {
     left: 10px;
   }
 
+  .columns-area {
+    direction: rtl;
+  }
+
   .column-header__buttons {
     left: 0;
     right: auto;