Attachments in alphabetic order in groups

[MasloMaslane]

I changed #388, so that attachments are sorted in alphabetic order in groups

[A-dead-pixel]

The Downloads view in problem sites should also have sorting for additional_files. The group ordering seems to already be present there. https://github.com/sio2project/oioioi/blob/a629e06f17647ee5d3ca333bec683e0a60dad295/oioioi/problems/problem_site.py#L136-L139