* Allow taking table prefix from env var
* FIX: remove unused column references
The columns `filedata` and `extension` are not present in a v4.2.4
database, and they aren't used in the method anyways.
* FIX: report progress for tables without imported_id
* FIX: effectively check for AR validation errors
NOTE: other migration scripts also have this problem; see /t/58202
* FIX: properly count Posts when importing attachments
* FIX: improve logging
* Remove leftover comment
* FIX: show progress when exporting Permalink file
* PERF: stream Permalink file
The current way results in tons of memory usage; write once per line instead
* Document fixes needed
* WIP - deduplicate category names
* Ignore non alphanumeric chars for grouping
* FIX: properly deduplicate user emails by merging accounts
* FIX: don't merge empty UserEmails
* Improve logging
* Merge users AFTER fixing primary key sequences
* Parallelize user merging
* Save duplicated users structure for debugging purposes
* Add progress logging for the (multiple hour) user merging step