This is pretty minor cleanup, but it makes it a little more
explicit what we're writing to the shard file, and it allows
us to use a more specific mypy type when calling
floatify_datetime_fields.
We no longer have an in-process code path to export
UserMessage rows. We want to only maintain the
subprocess code, which we'll always use in production,
and which will work fine in dev.
I also fixed some small things like removing unnecessary return
statements, and adding a TODO.
In some cases I explicitly cast stuff at run-time to set() or
str() to appease mypy, as well as make it clear to somebody
reading the code that the callee might not respect ordering
or tolerate unicode.
The previous export tool would only work properly for small realms,
and was missing a number of important features:
* Export of avatars and uploads from S3
* Export of presence data, activity data, etc.
* Faithful export/import of timestamps
* Parallel export of messages
* Not OOM killing for large realms
The new tool runs as a pair of documented management commands, and
solves all of those problems.
Also we add a new management command for exporting the data of an
individual user.
Steve Howell
Tim Abbott