Handle Undo from Misskey

Right now Misskey users were able to react, but couldn't remove their reactions.
delegates `Undo` for a `Like` to `undo_emoji_react` when there is no favourite found.

(Misskey `Like` activities can still create a fav when the emoji tag is invalid, I don't see the point though)

Signed-off-by: Plastikmensch <plastikmensch@users.noreply.github.com>

app/lib/activitypub/activity/undo.rb

@@ -110,13 +110,15 @@ class ActivityPub::Activity::Undo < ActivityPub::Activity
     if @account.favourited?(status)
       favourite = status.favourites.where(account: @account).first
       favourite&.destroy
+    elsif @object['_misskey_reaction'].present?
+      undo_emoji_react
     else
       delete_later!(object_uri)
     end
   end
 
   def undo_emoji_react
-    name = @object['content']
+    name = @object['content'] || @object['_misskey_reaction']
     tags = @object['tag']
     return if name.nil?