Liveness probe

Administrators only. Takes away what the import made, and nothing that people have done since. An issue is erased for good (not sent to Recently Deleted) only if nobody has touched it: it was not edited after the import finished, has no comment from anyone but the import, no live attachment and no link added since. Such an issue stays, with the comments the import gave it, and is counted in `keptIssues`. Comments the import added to issues of an earlier import are removed unless someone edited them. The ledger rows of erased issues go with them, so the same file can be imported again, with a corrected mapping, as if the first run had not happened. Statuses, labels and projects the import created are kept. Allowed for 7 days after the import finishes (`undoableUntil`). A 409 says why not: `not_undoable` (not finished), `undo_expired`, or `already_undone` (also what the loser of two simultaneous undos gets). Recorded on the security log.