Move screening-schema into DPG-Standard repo

[lacabra]

screening-schema.json in unicef/publicgoods-candidates currently defines the set of questions that operationalize the standard defined in DPGAlliance/DPG-Standard.

The standard is defined in standard.md, and operationalized in standard-questions.md, yet these are three different files that have to be manually updated to keep them in sync. This is highly inefficient and prone to errors.

The schema should be the source of truth, and from there generate any other related files where the same information is presented in a more human-readable way.

This task involves:

• Move screening-schema.json into DPGAlliance/DPG-Standard repo
• Add DPGAlliance/DPG-Standard as a submodule to unicef/publicgoods-candidates
• Adjust all the paths accordingly in unicef/publicgoods-candidates where screening-schema.json is used (e.g. CI)