UI to annotate a document with structured entities

[pudo]

Have a split-screen UI where a user is shown a document (e.g. PDF, email, ...) and a form where they can create new entities (such as Ownership relations), events, companies, people which are then linked to the document (and page within) as a source.

[pudo]

@iaincollins we discussed this, I'm actually wondering if it may be an interesting starting point for the case mgmt.

[pudo]

Requires #457.

[jigsawsecurity]

Being able to make notes about a document would be a great feature.