Parent/child documents

[roman-yagodin]

Most documents are just links to single file. But sometimes we need an ability to split file in parts (appendixes) or provide extra links like signatures, etc.

This will replace #30 as having separate entities for document and document file/link is an overkill.

And also leverage the need in #116.

But still, #116 would have sense to implement later, as every part of multipart document would need some kind of digital signature attached. But for now we can attach signature as a child.

[roman-yagodin]

Operations on parent document should apply (directly or indirectly) to its children.

Deleting parent document should promote its children? This will need FK rule. Also can just disable deleting of documents which have children attached.

Importing parent document from another module should also import all its children.

[roman-yagodin]

Child documents can be displayed in separate column or on separate lines just below parent document. The column display is most simple one to implement. Some presentation setting could be introduced later.

[roman-yagodin]

To preserve document relationships on export/import, we will probably need to export entity ids as GUID/UUIDs.