Add alt text field to Item Carousel widget

[alundgard]

• To each individual item container, add an HTML textarea with label Alternative text and placeholder message “Enter alt text for this item…”.
• Add a Decorative checkbox for indicating decorative images. Checking the box grays-out the alt text field (sets the “disabled” attribute).

• The current default functionality for this widget is to use the item’s metadata title as alt text.
• If a curator enters new alt text in the field (or checks the Decorative box) and clicks Save Changes, replace the current alt text with the new text.
• If a curator deletes previously-entered alt text (or unchecks the decorative box) and clicks Save Changes, restore the default alt text (item’s metadata title) and the placeholder field message.
• If there is existing user-written alt text, and a curator checks the Decorative box, hide the existing alt text from view, disable the field, and set the image alt to null. If the curator then unchecks the Decorative box, re-enable the field and repopulate the field with the previously-written alt text.