Add ratings for study documents.
Created by: NotSpecial
Add a rating for study documents, via a new endpoint studydocumentratings to rate, and a field rating to display the result.
We do not simply return the average vote, which can be quite far off, since we don't have many votes in general.
Instead, we return the lower bound of a statistical confidence interval.
On the one hand, it is useful, but actually I thought it's a neat approach, and since it is not much more complicated than an average rating I went for it^^ (The actual formula is a bit more complex, but thats it)
While I was updating the studydocs, I also cleaned up the old hooks. It still included a hook that does not exist in Eve anymore (since a few versions ago, there are no separate hooks for creating one or multiple items).
Closes #89