RWS: fix error when deleting submissions

Commit d89e43b03c468571f08267e26c97e897c3e9ac98 tried to remove a couple
of ugly workarounds but introduced an issue: delete_subchange tried to
access a submission that could already have been deleted. This is now
fixed, but nevertheless the bug showed the shortcomings of the current
data management.
Fix encoding issue in CMS

Pickle was causing issues when a file with non-ASCII characters was
submitted. Since pickle should always output str (both in the ASCII
format and in the binary format), opening the file in binary mode seems
the correct thing to do. And, indeed, it fixes the bug.
Fix bug in CWS

Sometimes downloading a file from CWS (for example a statement)
triggered a 500 HTTP error because a division by zero happened. I don't
know if it's because the timer had a low resolution or because the
transfer did really happen blazingly fast (or because the data was just
written to a buffer), but removing the speed computation surely fixes
the issue.