V5.3.1: SEO-related improvements and bug fixes

Changed

• Document how to set up the robots configuration for single view pages (#2598)
• Make the upgrade wizard for the separate billing address repeatable (#2558)
• Document that the slug field should to be enabled for editors (#2556)
• Make the object type for events a drop-down (#2551)
• Make the event slug field nullable (#2546)
• Make the event slug field shorter (#2539)
• Update the email CSS to simple.css v2.2.1 (#2543)
• Drop the UID from the event record slug (#2537)

Fixed

• Always provide the canonical URL for the single view (#2596)
• Use the correct DirectPersist trait from oelib (#2593)
• Also set/update the slug if the slug field is not shown (#2591)
• Ensure the upgrade wizard generates unique slugs (#2565, #2566)
• Make the slug generation upgrade wizard repeatable (#2557)
• Fix the slug generation for event date records (#2552)
• Also create slugs if the event or topic is hidden or timed (#2548)
• Add a maximum length to the seminar title in the TCA (#2544)
• Also create slugs for new records (#2537)