ETT-1137, ETT-454: focus indicator changes

[carylwyatt]

Two tickets from the a11y backlog pertaining to keyboard focus issues in page turner are included in this PR.

ETT-1137

The focus indicator around the container for the image/page and its toolbar was not very visible, and the focus indicator around the page itself was completely obscured. I updated the styles for those elements and now the focus indicators are completely visible in most circumstances, and mostly visible if the image is bigger than its container (e.g. if the image is zoomed in or in flip view on a narrow screen).

A side quest during this work was to update the ARIA labels on the image captions so the screen reader announces the OCR text when the screenreader tabs to the scan/image/page.

ETT-454

In thumbnail/grid view, only the first image in the viewport was reachable via the keyboard-- it essentially had a focus trap. You can see this in production currently if you try to move from the first image in the the thumbnail view with your keyboard, you can't do it.

This was a very simple fix once I found how that functionality was being controlled in page turner. I only had to update the focused conditional to be true for thumbnail view. I also tweaked the thumbnail focus indicator styles a bit to match the updates from ETT-1137.

To test

This is staged on dev-3. You might...

1. choose a volume to test in page turner (I provided Leo Lionni in the link above)
2. use your keyboard to tab to the image. the tab order is not consistent with the visual layout, but your keyboard should travel in this order: skip links (you could shortcut to the page content here) > navbar > reader toolbar at the bottom of the screen > sidebar > main reader container > page toolbar > scan
3. you could use your screenreader to see/hear the OCR of the image while in image view
4. using the VIEW button in the reader toolbar at the bottom of the screen, swap to flip view and tab through to both images and their toolbars. (possibly) observe how the focus indicator is partially obscured on those images.
5. swap to thumbnail view, tab through more than one image/page toolbar. there is a secret escape hatch on these images: if you press enter while focused on a scan, it will take you to scroll view of that image.

Let me know if any of these isn't working as expected! Thanks!

[kron-spar]

Can confirm that the tabbing works as described. I was also able to change the views and use the escape hatch when in the thumbnail view. This looks good to me!