Add alt text or other labels to graphical components used with GOCDB

[gregcorbett]

For accessibility, we should add alt text or other labels to graphical components used in GOCDB.

Predominantly, this will mean adding alt text to images - but at least the map on the portal's starting page would benefit from somehow labelling the relevant div.

Where a graphical component is purely decorative, we should use some sort of sensible empty element, such as the NULL alt text: https://www.w3.org/WAI/tutorials/images/decorative/

Useful links:

• https://www.w3.org/WAI/tutorials/images/decision-tree/
• https://stackoverflow.design/content/examples/alt-text/
• https://www.w3schools.com/accessibility/accessibility_labels.php