added email_sent field to student model, do not send mail twice if not needed

cntmanage/flightslot/models/students.py

@@ -19,6 +19,7 @@ class Student(models.Model):
 
     phone = modelfields.PhoneNumberField(
         null=True,
+        db_index=True,
         unique=True
     )
 
@@ -54,6 +55,11 @@ class Student(models.Model):
         Aircraft
     )
 
+    mail_sent = models.BooleanField(
+        null=False,
+        default=False
+    )
+
     def default_password(self) -> str:  # Maximum 4 digits for passowrd
         if self.pk:
             return f"{self.name.lower()[0]}{self.surname.lower()}{self.id % 10000}"