Be strict about similar email addresses for alerts

We don’t want a single person to have two accounts on an emergency
alerts service because it would let them circumvent the two eyes
approval process.

We can go some way to mitigating against this by stopping people using
common methods that email providers use to alias email addresses. These
are:
- being case insensitive
- being insensitive to the position or number of dots in the local part
  of an email address
- using ‘plus addressing’

We already prevent the first one, this commit adds normalisation which
strip out the second two before doing the comparision with the current
user’s email address.

app/main/forms.py

@@ -62,6 +62,7 @@ from app.models.roles_and_permissions import (
     roles,
 )
 from app.utils import merge_jsonlike
+from app.utils.user import distinct_email_addresses
 
 
 def get_time_value_and_label(future_time):
@@ -1060,6 +1061,10 @@ class InviteUserForm(BaseInviteUserForm, PermissionsForm):
 class BroadcastInviteUserForm(BaseInviteUserForm, BroadcastPermissionsForm):
     email_address = email_address(gov_user=True)
 
+    def validate_email_address(self, field):
+        if not distinct_email_addresses(field.data, self.invalid_email_address):
+            raise ValidationError("You cannot send an invitation to yourself")
+
 
 class InviteOrgUserForm(StripWhitespaceForm):
     email_address = email_address(gov_user=False)