This adds the Python copy of `hash_util.parse_narrow`. In the web app,
it will mainly be used in the import process later on. So, although it
has the same purpose as its frontend twin, there are differences:
- This doesn't convert a user-id-slug into a list of user emails. It
will instead parse it into a list of user IDs, as that is the preferred
form for those kinds of operators. It will also help in later operations
to remap the object IDs during import.
- To the same effect as the first point, operands can be an actual list
or int instead of a list or int as a string (e.g., "12,14,15" or "93").
- It has fewer validations than its frontend counterpart. It doesn't
look up the parsed object IDs for validity. This is partly because of
its main use case in import.
This prep commit adds a lib function 'is_same_server_message_link'.
This will be currently used while compressing quote and reply
in push notifications and later can be used at other places.
