Improve scheduling

• Introduce and visualize default slots
• Introduce interface to easily create/edit default slots
• Create new CVs/warnings based on default slots (moved to #163)
• Improve scheduler to use more of the available space (introduce "fullscreen" view?)
• Directly visualize violations and warnings in scheduling view
• Add reload button to scheduling view

To remove code duplication, this introduces a new version of the availabilities editor, too (this implements #93 (closed)). With this, all calendar inputs use a common fullcalendar v5. The new availability editor code can be reused for the default slot inputs.