Add stats to list of datasets

[mayhewsw]

Stats such as number of documents, number of annotated documents. Possibly percentage of annotated tokens, although that could be hard!

Will require updating the server.

[mayhewsw]

This will be useful to measure progress. As such, this should include, for each dataset, stats like:

• Percentage of docs annotated by at least one person
• Percentage of docs that have multiple overlapping annotators