Add profile edit form/view + modify mandatory models fields

• ProfileEditView allows connected user to modify its own profile according to its user type (guest student, tutor or volunteer).
• This view comes with tests.
• Many models fields was incorrecly mandatory.
• Fix birth_date field on User, which was allowed to be blank but not null on database level.

This commit introduces a restriction for a user to be of a single type: GuestStudent, Tutor, Volunteer.
To withdraw this restriction, one should rewrite ProfileEditView (the view for a connected user to change his data).