Figure modal a11y improvements

[StephDriver]

from #4194

• Sample 4, item 21. link to enlarge the figure is just 'figure 1' this needs to be more descriptive.
• Sample 4, item 22. having enlarged the figure, the modal is not navigable - no way to dismiss it.
• Sample 4, item 32. similar issues with Table 1 as with Figure 1 above. Cannot navigate to the close button, and end up outside the modal, but without it being dismissed.

Consider - if the enlargement has no difference in alternative text to the smaller version, making it obvious in the link so that users can decided whether to stick with the main article instead.

[joemull]

Refinement: We need to consider this primarily for the clean theme. The bug was discovered when testing on OLH theme, but clean theme should be prioritized. The JS utility we use for this feature is the same on both, though the HTML and CSS would of course be theme-specific.

We should consider whether there is an accessible bootstrap-provided utility for this that better integrates with clean HTML and CSS than the lightbox utility that we are currently using.

An even simpler alternative is the standard HTML element popover which should be considered if it is the simplest solution.