Prevent curation until ANNOTATION_FINISHED

[reckart]

Is your feature request related to a problem? Please describe.
A curator could start working while not all (or sufficient) annotators have marked a document as finished.

Describe the solution you'd like
Documents should become available to the curator only if the document has reached the state ANNOTATION_FINISHED.

If a document in principle falls back to ANNOTATION_IN_PROGRESS even NEW but the curation has already started (e.g. because a new user was added to the project or because the target number of annotators in the project changes for dynamic workflows), the curator should maybe be shown a warning when curating these documents.

It could also be considered to implicitly block the document for any new annotators if the curator has already started.

Additional context

• Cf. Documents already curated still appear in annotators' list webanno/webanno#1010
• Cf. Allow curation only after all annotators to who a document was assigned marked it as complete webanno/webanno#135