Replace forms by fieldsets

[bakura10]

Currently BaconUser uses form. However, forms are not reusable while fieldset are. This means that if someone want to have another form with a user, he has to duplicate all the fields.

We need to change that so we have a UserFieldset instead, and multiple forms (RegistrationForm, LoginForm…) that reuse the fieldset, and set the validation group accordingly.

[DASPRiD]

This sounds a little bit like overkill to me, let's discuss that on IRC.