Accessibility warnings

[queenvictoria]

I intend to come back to this issue and submit a pull request if I have time.

I see that there is an outstanding issue for this in the upstream library
dimsemenov/PhotoSwipe#1503

We have the opportunity in this addon to fix this. The code is pretty straight forward. Within the empty button we include something like

          <span class="sr-only" aria-hidden="true">Close slideshow</span>

And then in css

.sr-only { display: none; }

As I say--I'll circle back to this if I get time.

[queenvictoria]

Also we probably need to support alt tags too.
dimsemenov/PhotoSwipe#1423

[queenvictoria]

Instead of the sr-only <span> we can do this:

<button aria-label="Next"></button>