Review required features

[markwhiting]

Required is quite simple now and has some failure modes as a consequence. For example checkboxes can't have required because they are seen as independent inputs (so all are required) e.g., #106, and we have no way to enter nuanced validations such as a number between 1 and 10.

This requires us to make a plan around how to deal with some popular and edge cases for required, and at least the following:

• fix required for checkboxes so that it has a default behavior like "at least one checkbox is checked"
• consider how required might work for each input type and make a plan for possible validation rule schema we could use for each one. Document this ideation in the thread here and get agreement on a strategy with @markwhiting before implementing.
• once approaches are decided upon, implement them
• create documentation to describe how each requirement schema works
• create tests to check that each required mechanism is working and robust (including ones that do not require new validation schema)

This is done when all steps above are complete and all other tests pass.