New signup method "complex requirements"

[jeriox]

This is meant to be a general purpuse signup methods for complex requirements, esp. dealing with various counts of different qualifications and people.

Further specification in the design blog.

[felixrindt]

The blocks editor is prepared by now. Besides the main open task of implementing it as a shift structure, these are some additional points to consider:

• [ ] Vue production version
• [ ] Vue CSP
• [ ] unify how required qualifications are rendered accross shift structures