[accessibility] - missing labels

The buttons to accept / decline a challenge do not have a label, thus making it hard to a screen reader access them.

How to reproduce:
1. Issue a challenge to an user.
2. the user, with a screen reader turned on, should go to notifications.
3. The challenge will be there with user, rating, time control. Two buttons will appear, and they do not have a label.

Hello,

That should fix it github.com/ornicar/lila/commit/e7ef1c91c6e9a18eacf697571bb0019b37b0e337

thank you!