feat: image url migration

registrations can now store image urls, which will be shown at view.html

WebApp/DTOs/EventSummaryNoErDto.cs

@@ -10,6 +10,7 @@ public record class EventSummaryNoErDto(
     [Required] int OrganisationId,
     [Required][StringLength(50)] string Title,
     [StringLength(500)] string Description,
+    string? ImageURL,
     [Required][StringLength(100)] string Location,
     [Required] DateTime? EventDate,
     ICollection<EventSkill> EventSkills