fix/Change delete_email Receiver to be Case-Insensitive (#6070)

`UserEmail.email` is case sensitive but `Email.email` is case-insensitive.
We should only delete `Email` when there are no case-insensitive matching `UserEmail`

Author: ehfeng

src/sentry/receivers/email.py

@@ -16,7 +16,7 @@ def create_email(instance, created, **kwargs):
 
 
 def delete_email(instance, **kwargs):
-    if UserEmail.objects.filter(email=instance.email).exists():
+    if UserEmail.objects.filter(email__iexact=instance.email).exists():
         return
 
     Email.objects.filter(email=instance.email).delete()