message_edit: Check previous message content to prevent races.

Similar to group based setting values, we expect
the client to send the previous content alongwith
the edited content to the edit message endpoint.
We reject the request incase the previous content
doesn't match the current message content, which
could happen in case two users simultaneously edit
a message - which will be implemented in #33051.

zerver/lib/utils.py

@@ -1,3 +1,4 @@
+import hashlib
 import re
 import secrets
 from collections.abc import Callable
@@ -68,3 +69,7 @@ def get_fk_field_name(model: type[models.Model], related_model: type[models.Mode
     assert len(foreign_key_fields_to_related_model) == 1
 
     return foreign_key_fields_to_related_model[0].name
+
+
+def sha256_hash(text: str) -> str:
+    return hashlib.sha256(text.encode()).hexdigest()