Allow string fields to offer supporting urls

[mccalluc]

Filing as draft.

• Any preferences on the names? I think "truth basis" was mentioned, but I may be misremembering?
• Is the idea that in the end fields will be required to have a source, or will it remain optional?
• Would it ever be multi-valued? ie, multiple supporting references?
• Should we require the source to be a URL, or do we need to allow non-URL sources?
• Is it ok for it to be the full URL, or would you prefer to use some kind of short code, and then a list of all the codes and the full urls would allow it to be expanded? If every field needs to list a source, and it's mostly from the same PDF, it's going to get very redundant... but adding an extra referencing step would add complexity. Another option would be the YAML reference syntax.
• With this introduction of a repeated subschema, would it make sense to treat the *_description fields in a similar way?
• In my imagination, we would finish the work here before pinging the UI team.... but they could start doing some work in parallel. Preference? And who handles communication with them, to make sure this whole thing comes together?
• What are the other schema changes required? I would like to be able to see the whole picture before committing to a lot of work here.

[tercer]

Previously the term "Trust Basis" was proposed, and the current term is "Evidence Source".

Here's the current language from the draft:

[tercer]

Here's the current schema definition in the paper:

[mccalluc]

• Closing in favor of Support evidence source #172