Validation for form specifications

Single form validation:

1. Use JSON schema to perform basic structural validation
2. Check that all fields, choices and questions have unique `slug`s
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