get token count per doc in doc browser

[amir-zeldes]

The document browser should show the amount of tokens in each document by default in a separate column.

[thomaskrause]

This information is currently not stored in the database. Since we don't want to query it each time we would need to extend the corpus_stats table to include also documents. Since the table is also used for calculating the offset IDs this is not a trivial change.