This fixes an issue where passing a path like `~/exports/foo` would
result in a `~` directory being created and the export/import not
working correctly.
Significantly tweaked by tabbott because:
* Argparse was already handling the early checks
* Splitting the bottom loop into two loops means we validate all the
input before trying to run actual import code on anything.
* The argparse documentation was confusing about whether the paths
should be files or directories.
When you're importing with --destroy-rebuild-database, we need to
check subdomain availability after we've cleared out the database;
otherwise, trying to reuse the same subdomain doesn't work.
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.