Validation and filter feature

There have been a number of issues reported (linked in #882) where it seems that user's data files have been corrupted. Some of these things should be fixed on the frontend, so that it makes sure invalid data is not sent. However, the backend should also check, e.g. by doing a RDF validation as suggested in #882, but we could also imagine SHACL validation, etc. If Inrupt becomes a large POD provider, it may also come with legal requirements.

Also, validation may not only be a boolean accept or error, but possibly also filtering to accept valid parts. For v.next, we need to have an architectural element that does this, but we may need to address parts of this problem already for 5.0.0.

Consider this a super-issue for discussing what should be in 5.0.0, and if we should try to make that reusable in v.next. Also, we may discuss if we do not attempt to solve it on the backend and refer to frontends to do it.

Start by reading the reports linked in #882 and the discussion about RDF and SHACL validation. Clarify whether validation belongs in the frontend, backend, or both, and define the scope for 5.0.0 versus v.next. Done means an agreed reusable architecture and clear requirements for rejecting or filtering invalid data.