This allows to avoid repeating the same pattern of retrieving the last snapshot
for a given origin.
Note that this also makes the new function return a Snapshot model object as
well.
Impacts only swh-web which I'll deal with
```
(.../swh-environment) $ grep snapshot_get_latest */swh/**/*.py | grep -v "\btest" | grep -v "swh-storage/swh"
swh-web/swh/web/common/service.py: snapshot = storage.snapshot_get_latest(origin, allowed_statuses)
````
Related to T2310