[rel-db] Initial data import

[rrenkert]

We need to rebuild our import for initial data sets. For testing purposes there are files (SQL) containing basic data in the meta-repo but for dev, demo and production we need a way to import different sets of data.
We already discussed this and the format should stay in JSON.

[luisa-beerboom]

@hjanott merged a PR that fixed the initial_import action last week so this can be closed. (#3152)