V5.7.0: Webinar URL, BE usability features and deprecations

Added

• Add a "duplicate event" button to the BE module (#3469, #3482, #3485, #3487)
• Add a field for additional text for the thank-you emails
  (#3453, #3456, #3459, #3465)
• Add Venue.fullAddress (#3399)
• Include the webinar URL in the thank-you email (#3446, #3452, #3462)
• Include the webinar URL in the calendar invite (#3445, #3452)
• Add a field for the webinar URL to events (#3380, #3384, #3451)
• Add more venue fields: contact person, email, phone number (#3362, #3363)
• Add event formats (on-site, hybrid, online) (#3343, #3361)
• Show a flash message after deleting an event or a registration (#3340)
• Allow viewing event UIDs in the TCEforms (#3337)

Changed

• Do not use the event subtitle as calendar invite description anymore (#3398)

Deprecated

• Deprecate some RegistrationManager methods (#3491)
• Deprecate some functions from the old Event model (#3490)
• Deprecate some functions from LegacyEvent (#3489)
• Deprecate icons for categories (#3488)
• Deprecate Event.getRegisteredSeats() and Event.hasEnoughRegistrations()
  (#3443)

Fixed

• Do not duplicate registrations when duplicating an event (#3479)
• Do not provide the LanguageService via DI anymore (#3467)
• Avoid empty paragraphs in thank-you email HTML (#3461)
• Do not create calendar invites without a defined start and end (#3427)
• Avoid calendar invite location for multi-venue events (#3400)
• Improve some type annotations (#3347)
• Always use strict comparisons (#3336)