[ENG-7922] chunky delete_pretrove_data (and --chunksize param)

[aaxelb]

update delete_pretrove_data management command to accept --chunksize param (default 1024) and delete data in small chunks (and from one sourceconfig at a time) instead of deleting all-at-once

local (contrived) example:

root@eac74ef154f9:/code# python manage.py delete_pretrove_data
pre-trove source-configs with deletable rawdata:
	system.v2_push
really DELETE ALL raw metadata records belonging to these source-configs? (y/n)y
system.v2_push: deleting all rawdata...
system.v2_push: deleted 1024
system.v2_push: deleted 1024
system.v2_push: deleted 1024
system.v2_push: deleted 1024
system.v2_push: deleted 904
system.v2_push: done; deleted 5000
deleted 5000 items
root@eac74ef154f9:/code# python manage.py delete_pretrove_data
nothing to delete

ENG-7922

[coveralls]

coverage: 79.141%. remained the same
when pulling 3f158c6 on aaxelb:fix/chunky-delete
into 9bfea80 on CenterForOpenScience:release/25.2.0.