Ability to add & edit alt-text for uploaded images or other non-SDR content

[caaster]

This is a multi-faceted a11y requirement. As an exhibit creator, I would expect the following behavior:

• When I upload a single non-SDR image to my exhibit (Curation > Items > Add items > Upload item), there should be a field automatically provided for me to add alt-text. This is the alt-text that I would expect to be available to screen readers on the Item details page (OpenSeaDragon viewer).
• When I bulk upload non-SDR images to my exhibit (Curation > Items > Add items > Upload multiple items), there should be a field automatically provided for me to add alt-text for each image. This is the alt-text that I would expect to be available to screen readers on the Item details page (OpenSeaDragon viewer).
• When I use a widget to display an image I have uploaded using the Upload item or the Upload multiple items options, I need to be able to edit the alt text (that I provided when I did the initial upload) on each page to accurately reflect the context on each page. The widgets that might be used include: Item row, Item carousel, Item embed, Item slideshow, and possibly Item grid. I may need to make a different alt-text edit to each page where the image is displayed so that the appropriate context can be provided.
• When I use a widget to display an image that I have uploaded using the Item row widget, I need the ability to add alt-text for that image.
• Further investigation needed for the iframe and video widgets, where the ability to add alt-text may also be required. Interesting use cases exist for using the iframe widget to display ArcGIS story maps or Matterport files on a page.
• Further investigation needed for all alt-text requirements for IIIF-uploaded images.

[corylown]

Some context from an older related issue on this topic: https://github.com/sul-dlss/exhibits/issues/819