Captcha Math question needs to be more accessible

With this still being a problem in the 2.x branch I'm going to update the issue appropriately, create an issue fork, and reroll adriancruz's patch from #2 for the latest 2.x release.

Also updating this to a major bug due to the fact the module as written will not pass accessibility tests.

I can update the issue fork to add a span around the subset of the label containing the equation in order to satisfy the failed test but wanted to get maintainer feedback first.

Re-rolled this patch for the 2.x branch

I can confirm that we just had this issue reported to us during a mandatory accessibility audit ;-)

Thanks for the patch!

As a side note, considering this makes the captcha inaccessible, this prevents people from using forms, which may be quite important.

There I'm raising the priority of the issue.