Add "exposable_reactions" to features, if showing reactions

lib/pleroma/web/mastodon_api/views/instance_view.ex

@@ -83,7 +83,10 @@ def features do
         "safe_dm_mentions"
       end,
       "pleroma_emoji_reactions",
-      "pleroma_chat_messages"
+      "pleroma_chat_messages",
+      if Config.get([:instance, :show_reactions]) do
+        "exposable_reactions"
+      end
     ]
     |> Enum.filter(& &1)
   end