Insert stats about unknown words into `statistics_documentstatistic`

[egli]

The old system used to insert the number of total words and unknown words into a separate table statistics_documentstatistic

[egli]

Well unfortunately the commit above doesn't work as it does a select with a limit of 20, so the number of unknown words is always 20 😞 . There is a commit (25354d5) which rectifies this problem, but it is ugly, so I moved it to a branch.

Let's look at this again after the db migration to aggregated tables.