Refactor Signup Method: Signup Flow and Shift Structure

[felixrindt]

• [x] QualificatioMixStructure
• [x] race condition
• [x] what happens when deleting a Qualification configured in the shift structures
• [x] check PDF export
• [x] update tests to work again
• [x] add new tests
• [x] update plugin dev docs
• [x] update user docs and make sure it's linked correctly
• [ ] profiling and performance improvements
• [x] display flow/structure descriptions
• [x] migration
• [x] delete old plugin

[coveralls]

coverage: 87.346% (-0.5%) from 87.828% when pulling 826c58b1440b25185ff38e9f5ceeb3be9391d526 on signup-flow-refactor into 355949326f7d6cd63709d40d3748dbe1756bebe4 on main.