Keep track of schema and ontology import resolution

[imsdu]

Motivation
Schema imports can be difficult to manage as a sub-schema or an ontology can be imported several times (directly or transitively)

In order to handle and validate them properly, we need to keep track of how it has been resolved.

Acceptance criteria

1. For each imported schema/ontology, we keep track of its project, its id and its revision at the time of the schema creation/update
2. We also keep track if an schema/ontology is imported directly or transitively through another schema