It was already used in two places (views.py and photon connector), and
wrongly (photon connector was ignoring some errors for no reason).
META_SCHEMA manipulation is removed and will be replaced by a
normalization of the schema to remove lazy strings in a later commit.
A new JSONValidationError subclass of APIError is introduced.
* flatten/unflatten JSON document
* flatten JSON schema (to help users in producing flattened JSON
documents, not to validate, validation must be done through
unflattening then validating using the original JSON schema)
Benjamin Dauvergne
Emmanuel Cazenave
Frédéric Péters
Nicolas Roche