Change user registration to prevent fake accounts

[roskakori]

User story

• As new user I want to be able to register with reasonable effort and within reasonable time in order to be able to contribute to pyGRAZ quickly.
• As organizer I want all users to be real users in order to have an estimate on the active user base.

Background and motivation

See #25.

Goals

TBD; Possible new process for registering new users: Send email to one of the organizers. Or make an application form, which will be reviewed by the organizers, who then create the user manually.

• The ReCaptcha is gone.