settings: Add permission to enforce unique names in realm.

Previously, users were allowed to signup or change their names to
those which already existed in the realm.

This commit adds an Organization Permission, that shall enforce
users to use unique names while signing up or changing their
names. If a same or normalized full name is found in realm,
then a validation error is thrown.

Fixes #7830.

zerver/forms.py

@@ -180,6 +180,11 @@ class RegistrationForm(RealmDetailsForm):
     )
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
+        # Since the superclass doesn't except random extra kwargs, we
+        # remove it from the kwargs dict before initializing.
+        self.realm_creation = kwargs["realm_creation"]
+        self.realm = kwargs.pop("realm", None)
+
         super().__init__(*args, **kwargs)
         if settings.TERMS_OF_SERVICE_VERSION is not None:
             self.fields["terms"] = forms.BooleanField(required=True)
@@ -211,7 +216,9 @@ class RegistrationForm(RealmDetailsForm):
 
     def clean_full_name(self) -> str:
         try:
-            return check_full_name(self.cleaned_data["full_name"])
+            return check_full_name(
+                full_name_raw=self.cleaned_data["full_name"], user_profile=None, realm=self.realm
+            )
         except JsonableError as e:
             raise ValidationError(e.msg)