Codice Fiscale should be validated and check if unique

[giobber]

Description

"Codice Fiscale" field should be unique as it represent the unique ID of a person in Italy There are two main problems:

• CF are not always unique, this was solved by using omocodes but is common online for users to avoid using his personal omocode and instead using the not omocode CF to avoid problem with too much simpler validation
• A lot of people insert wrong CF because of typos, memory, laziness (see #50 for possible solutions)
• For foreign person (mainly immigrants) is common to use not so clean data when registering in Italy...so a lot of omocodes and strange CF
• No good database of province is available, python-codicefiscale has a good solution but in general is a best effort

Solution

• [x] Set Member.cf as unique=False (to permit foreign and strange CF to pass)
• [x] Add field `Member.skip_cf_valid = models.BooleanField(default=False)
• [x] Add validator in Member to check if Member.cf is valid (see #50 )
• [x] Add validator in Member to check if Member.cf is unique unless Member.skip_cf_valid=True

[giobber]

After more thinkering: a lot more simple solution is:

• [x] Set Member.cf.unique = False (it is already)
• [ ] Inside Member form or Member subscription view add check for uniqueness of cf value
• [ ] Inside Member form or Member subscription view add checkbox to skip uniqueness check if that error was previously raised