feat(db): database structure for contexts

[blizzz]

Implements the DB structure as proposed in #812 with these two differences:

• for consistency, I renamed "resource" to "node" how it is already named in the existing share table
• for it is not possible to have a column being nullable and part of the primary key, in navigation i changed the user_id to not be nullable and have the default set to an empty string. The intended logic therefore is changed that the default is an empty string, instead of null. Otherwise, same behaviour is envisioned.
• owner_type is now an int, to be used with constants to be introduced, as discussed

Dedicated indices will be added later on once the queries are standing. Ideally prereleases are not shipped without them being done.

ℹ️ this PR is opened against a feature branch, feat/contexts freshly forked from main. This is because 0.7 is not completed and branched off so far. We can merge it to main either after 0.7 is branched off.

Contributes to #311