add created_via to participant entity

ref #211

src/models/entities/Participant.ts

@@ -75,6 +75,14 @@ export abstract class Participant {
   @IsEmail()
   email?: string;
 
+  /**
+   * how the participant got into the system
+   */
+  @Column({ nullable: true })
+  @IsOptional()
+  @IsEmail()
+  created_via?: string;
+
   /**
    * Turns this entity into it's response class.
    */