No support for a login/sign up with a personnummer

[PierreMesure]

Right now, the website just uses the e-mail address as an identifier.

In Sweden, it is usual to use your personnummer. This would make it easier to check people's identity and would be a good first step if the platform is to support SPAR and BankID as an authentication/e-voting method in the future.

This task can actually be divided in the following actions:

• add a new column to the User model
• add a field during the sign up process.
• add a field in the profile page
• allow users to login with their personnummer.
• fix all the tests!

The changes to the verification process are a bit heavier and will be done in another task (probably related to SPAR)