Member model improvements

[giobber]

• Add third option to gender option (Male|Female|Other)
• re-add name, surname here (does not depend on user, teacher will have a display_name, display_surname)
• Codice Fiscale / Social ID
  • set max_length of CF to 20/24 (validity will be check in form)
  • Add method to check social_id formal validity and against other personal data (see Add check for SocialID/CodiceFiscale #81)
  • Add social_id validator (see Member | Check personal data against codicefiscale #64)
  • Move CF check in MemberForm
• Move phone check in MemberForm
• Add a model to handle DocumentType
• Remove document_expires default (imported data will use a default)