Clarifying link purpose for screen readers

[jaguillette]

Our university IT's accessibility group took a look at Omeka as provided to our community, and identified a few issues. One is related to WCAG 2.1 success criterion 2.4.4: Link Purpose. Essentially, on the admin pages that list items and collections (also exhibits, but that's not this repo), screen readers can extract all of the links and get a repeated sequence of "Details", "Edit", and "Delete" links removed from the context of what they act on.

I'm working on a fix to add the item title to each of these links, but to also hide it visually so that it's only picked up by screen readers. I'm starting from the new-admin-theme branch so that if these changes ought to be rolled into the upcoming admin theme refresh, they can be.