Expand DDL validation

[Lisser]

• Try to forge a GraphQL DDL schema that breaks things: (e.g. override built-in types)
• Write unit tests
• Write validators

Good approach: whitelist approach instead of blacklist approach (disallow everything at first, then introduce functionality)

[Darwinkel]

Progress tracker:

• Write a couple of both syntactically and logically malformed GraphQL DDL schema's (in ddl/schemas/)
• Write unit tests which assert that the malformed schema's are correctly rejected (in tests/)
• Write boolean whitelist validator logic which checks that a schema is valid (in ddl/ddl.py)