Switched emails to being mandetory for users

ref #93

src/models/entities/User.ts

@@ -25,9 +25,10 @@ export class User extends Principal {
   * The user's e-mail address.
   * Either username or email has to be set (otherwise the user couldn't log in).
   */
-  @Column({ nullable: true, unique: true })
+  @Column({ nullable: false, unique: true })
   @IsEmail()
-  email?: string;
+  @IsNotEmpty()
+  email: string;
 
   /**
   * The user's phone number.