[Feature request] Implement indicating number of annotated documents (by the user)

Feature description

Feature request: implement indicating number of annotated documents (by the user). Each annotator sees how many documents are being annotated in the project, by him.

Please provide us (and other contributors) the evidence of the situation that you need this feature.

@icoxfog417 , real situation: our annotators ask project administrator, more and more: how many documents are being annotated by me? How many more documents should I annotate?

Related: #948.

Is this question solved？