Changes to consider "assignee" as a name field and not a text field

Following discussions under #485, "assignee" is to transfer from itemFields to creatorTypes.

Also, it is mapped to CSL's "recipient".

Edit: I pull it against "master" as it changes the SQL. I test it on my Firefox; seems to work very well. However, I guess there is some other changes to do regarding the SQL update, to transfer what was recorded in the former field into the new one.