This has no functional changes; we just replace the old hacky
assignment of functions with assignment of the upload backend to a
variable.
I'm not totally happy with this, because we end up having to copy the
type annotations of the three methods 4 times each, but this should
make it a lot easier to test the (non-default-in-tests) S3 backend
using end-to-end tests, which would have caught
13bac1cc2a.
I expect we'll iterate on the interface over time; ideally, I'd like
all the code that checks LOCAL_UPLOADS_DIR to be inside upload.py, and
primarily in these classes.
This commit adds the capability to keep track and remove uploaded
files. Unclaimed attachments are files that have been uploaded to the
server but are not referred in any messages. A management command to
remove old unclaimed files after a week is also included.
Tests for getting the file referred in messages are also included.
