models: Refactor get_fake_email_domain to take realm.host as arg.

This commit updates get_fake_email_domain to accept realm.host as
argument instead of the Realm object since we only use realm.host
to get the fake email domain.

This is a preparatory commit for the limited guest feature as we
would be sending the fake email of the message sender in message
event object to a guest user who cannot access the sender and
there we would need to compute the fake email.

zerver/lib/users.py

@@ -796,7 +796,9 @@ def get_cross_realm_dicts() -> List[APIUserDict]:
 
 
 def get_data_for_inaccessible_user(realm: Realm, user_id: int) -> APIUserDict:
-    fake_email = Address(username=f"user{user_id}", domain=get_fake_email_domain(realm)).addr_spec
+    fake_email = Address(
+        username=f"user{user_id}", domain=get_fake_email_domain(realm.host)
+    ).addr_spec
 
     # We just set date_joined field to UNIX epoch.
     user_date_joined = timestamp_to_datetime(0)