Globally defined types, schemas and rules

[Syndesi]

Many advanced use cases requires control over the schemas of individual types in Neo4j, MongoDB and especially Elasticsearch.

Furthermore with the increasing integration of Go's Expr Lang library, it is expected that custom validation rules for specific types / schemas are doable and useful enough to implement.

Acceptance criteria:

• Fully define feature.
• Allow type based schemas and validation rules.
• Allow changes to schemas. Might require database migrations.
• How should incompatible schema changes be handled? Rollback? Error? Execute anyway?
• Restrict schema changes to administrator-type users, requires Add user roles / flags #476 .
• Add endpoints for managing schemas.
• Any schema changes must be logged (who, when, what, ...).