Refactored signup

In an attempt to improve the structure of the code, I moved the sign up of both companies and students to the custom_users app. It was a difficult decision, but I finally made it because signing up actually creates a custom_user and not really a student or company in particular. The student and company then just decorates this custom user.

Also, it seemed quite weird to have one view that did not require that the user was logged in, in the two apps.