Validation and filter feature

首先阅读 #882 中链接的报告以及关于 RDF 和 SHACL 验证的讨论。明确验证应在前端、后端还是两者中进行，并确定 5.0.0 与 v.next 的范围。完成的标准是就可复用的架构达成一致，并明确拒绝或过滤无效数据的要求。

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.