User: Add function to get AP ids from nicknames.

lib/pleroma/user.ex

@@ -1441,4 +1441,12 @@ defmodule Pleroma.User do
       update_and_set_cache(cng)
     end
   end
+
+  def get_ap_ids_by_nicknames(nicknames) do
+    from(u in User,
+      where: u.nickname in ^nicknames,
+      select: u.ap_id
+    )
+    |> Repo.all()
+  end
 end