Validation for form specifications

[japsu]

Single form validation:

1. Use JSON schema to perform basic structural validation
2. Check that all fields, choices and questions have unique slugs
3. Check that all configuration directives (such as choicesFrom) have options that make sense

Multilanguage form validation:

1. Warn about fields, choices and questions that are not present across all language versions

Use case specific form validation:

1. If the use case requires specific fields to be present, warn if they are missing or incorrectly configured