Add "alt text" option for banner image

[djplaner]

Module banner images currently have a hard-coded (largely useless) alt text.

As suggested add a text area to allow entry of alt text for banner images.

[djplaner]

Design

Banner image component - add Alt Text element

• [x] title
• [x] help link
• [x] help page
• [x] text box

Component should also

• [x] add an empty altText value if one not there
• [x] keep the alt text free of any and all HTML - use sanitize-html

Representations should use the alt text

• [x] Griffith Cards
• [x] Horizontal Cards