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 adopt BankID as an authentication/e-voting method in the future.

This task can actually be divided in two:

• add a field during the sign up process and connect it to a new column in the database (or maybe a new table with a foreign key?).
• allow users to login with their personnummer.

[PierreMesure]

Support added. It is now necessary to write in a personnummer to create a user account (not for an organisation).
It is also possible to login with your personnummer.

What remains to be done:

• fixing the last tests that I broke with this new feature
• add more checks on the personnummer field (currently can enter any varchar value) (see Wikipedia and this article

[PierreMesure]

Moving the task to digidemlab/consul.