Investigate the support of HTML "alt" tags via Nunaliit's existing schema definitions

[ahayes]

Populating alt tags on supported HTML elements in Nunaliit is important to achieve our goals for accessibility. With much of atlas content being supplied by atlas builders and their users, being able to configure the attributes that form the atl content when developing schemas is important. Providing reasonable defaults when not specified would also be important.

For example, attributes in schema definitions could allow for a flag such as "includedInAlt" similar to the "includedInBrief" boolean option. This would allow for alt tags to be built from several attributes if needed. A good default behaviour might be to simply populate the alt tag with the brief when there is no specific configuration provided.

If digging into this section of the code, a bonus effort might be to devise a more flexible brief building syntax and use it for both brief and alt building. Maybe something like the conditional styling system.