backend, pgsql: new `overwrite_policy` node setting: /!\ pgsql schema needs to be updated /!\ this settings can be set currently to 2 values: - `original_publisher` (default), when only original publisher of an item can overwrite it (except node owner/admin) - `any_publisher` when any entity with publishing right can overwrite any item.

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);