fix n+1 queries with invite card

lib/memex/accounts/invites.ex

@@ -109,6 +109,19 @@ defmodule Memex.Accounts.Invites do
     )
   end
 
+  @spec get_use_counts([Invite.t()], User.t()) :: %{Invite.id() => non_neg_integer()}
+  def get_use_counts(invites, %User{role: :admin}) do
+    invite_ids = invites |> Enum.map(fn %{id: invite_id} -> invite_id end)
+
+    Repo.all(
+      from u in User,
+        where: u.invite_id in ^invite_ids,
+        group_by: u.invite_id,
+        select: {u.invite_id, count(u.id)}
+    )
+    |> Map.new()
+  end
+
   @spec decrement_invite_changeset(Invite.t()) :: Invite.changeset()
   defp decrement_invite_changeset(%Invite{uses_left: nil} = invite) do
     invite |> Invite.update_changeset(%{})