Feature/import data pdp

Fixes #261 (closed), #203 (closed), #254 (closed)

Also important commit that must be merged (or at least cherry picked) since it fixes and error on the Voucher Tables: 5e979b18

I think it's done unless we need to make more changes to the data itself, like, adding the flags etc. Which we can do on the website instead of using the program, unless we want a more general program for importing information. Right now I would consider the program basically ad-hoc for this specific case. Since some information must be hardcoded like the VouchersTypes or the format of the Voucher References to get the issue date and voucher type.

Added excluded property to studySubject model. Regarding this, I saw a comment which says: "Marking a subject resigned or deceased cannot be undone.". Why not? and, should resigned option have the same behaviour?