Allow recording documents as references only

[vildead]

Related to #385

File upload field in DatasetCreate modal box is now required and it is impossible to create an "empty" document record.

Define 2 types of documents

1. real document uploaded in DAISY
2. reference with URL

Flow:

1. User clicks on + button to add new document
   2a) either the modal box will have toggle on the top to choose from "Document upload" and "Document reference
   2b) or they will see small window over the + button with those options (but 2a would allow for more text to be added)
2. based on the selection, the form would either have the file upload field or URL/URI field (with simple validation)
3. The type of document record can be also visually different on the main page (project, contract)