Foreign key constraints in the backend datastores

[wood-push-melon]

Currently, there are no foreign key constraints added in the backend datastores' schemas. Probably we can consider adding some foreign keys in the users and includegroups tables. Take an example in the PostgreSQL backend:

ALTER TABLE users
    ADD CONSTRAINT fk_groups_users FOREIGN KEY (primarygroup) REFERENCES groups (gidnumber) ON UPDATE CASCADE;
ALTER TABLE includegroups
    ADD CONSTRAINT fk_parent_group FOREIGN KEY (parentgroupid) REFERENCES groups (gidnumber) ON UPDATE CASCADE;
ALTER TABLE includegroups
    ADD CONSTRAINT fk_child_group FOREIGN KEY (includegroupid) REFERENCES groups (gidnumber) ON UPDATE CASCADE;

This allows us to update necessary fields in users and includegroups tables whenever there is a change in group's gidNumber to not violate the data integrity.

And also we can evaluate if we need to add similar things for group deletion operations.

There might be additional actions required for some other databases: it seems users need to enable foreign key support in SQLite: https://www.sqlite.org/foreignkeys.html#fk_enable