Limit length of filename

S3 has a limit of 2kb for metadata:

> the user-defined metadata is limited to 2 KB in size. The size of
> user-defined metadata is measured by taking the sum of the number of
> bytes in the UTF-8 encoding of each key and value.

– https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html#object-metadata

This means we have a limit of 1870 bytes for the filename:
```python
encoded = 'notification_count50000template_id665d26e7-ceac-4cc5-82ed-63d773d21561validTrueoriginal_file_name'.encode('utf-8')
sys.getsizeof(b)
>>> 130
2000-130
>>> 1870
```

Or, in other words, ~918 characters:
```python
sys.getsizeof(('ü'*918).encode('utf-8'))
>>> 1869
```

app/utils.py

@@ -568,3 +568,8 @@ class GovernmentEmailDomain(AgreementInfo):
             ))
         except StopIteration:
             raise NotGovernmentEmailDomain()
+
+
+def unicode_truncate(s, length):
+    encoded = s.encode('utf-8')[:length]
+    return encoded.decode('utf-8', 'ignore')