Use the correct csrfmiddlewaretoken input to populate the ad-hoc preview form.

This is a regression from 7258e049f80743796ca7907b05e8022a36d832b8. Fixes bug #1054294.
Merge pull request #2671 from jezdez/fix-wiki-preview

 Use the correct csrfmiddlewaretoken input to populate the ad-hoc preview form.