R1:8f81b34ea1a0

Improve lispum generation of pastes

Summary: Fixes T8482, or something. I can't actually repro that but I think it should be fixed either here or earlier

Test Plan:
```
$ ./bin/lipsum generate paste
GENERATORS Selected generators: Pastes.
WARNING This command generates synthetic test data, including user accounts. It is intended for use in development environments so you can test features more easily. There is no easy way to delete this data or undo the effects of this command. If you run it in a…