Prior to this change, there were reports of 500s in
production due to `export.extra_data` being a
Nonetype. This was reproducible using the s3
backend in development when a row was created in
the `RealmAuditLog` table, but the export failed in
the `DeferredWorker`. This left an entry lying
about that was never updated with an `extra_data`
field.
To fix this, we catch any exceptions in the
`DeferredWorker`, and then update `extra_data` to
encode the failure. We also fix the fact that we
never updated the export UI table with pending exports.
These changes also negated the use for the somewhat
hacky `clear_success_banner` logic.
It seems possible that attempting to export large organizations could
result in high resource consumption that justifies having a technician
manage the exports manually.
This feature is intended to cover all of our ways of exporting a
realm, not just the initial "public export" feature, so we should name
things appropriately for that goal.
Additionally, we don't want to include data exports in page_params;
the original implementation was actually buggy and would have.