Add new input fields for user registration

Description

This MR introduces new checkboxes users have to consent with before registering (won't result in any persistent changes in the database) and breaks down the name field of user into first_name and last_name. Furthermore a new optional field organization` is introduced.

Closes #62 (closed) #63 (closed)

TODO

• adjust checkbox texts to correct ones as soon as dirk sends them
• provide route for changing user information (or follow up?) – see #4 (closed)