Schema files of data domain models could not be loaded correctly

[ealtendorf]

1. The JSON schema version is not explicitly specified in most data domain model definition files:

"$schema": "http://json-schema.org/schema#" should be changed in:
"$schema": "http://json-schema.org/draft-07/schema" (or some other version)

Otherwise an exception is thrown:
Caused by: org.everit.json.schema.SchemaException: #: could not determine schema version: no meta-schema is known with URL [http://json-schema.org/schema#]

2. Some links reference a file in GitHub, but not as a raw.githubusercontent.com link, so that these references also could not be loaded correctly.

[dagnelies]

+1