FEATURE: Add uploads:batch_migrate_from_s3 task to limit total posts migrated at once (PR #9933)

Allow limiting the number of migrations to do at once, both to do migrations that have impact limited to multiple off-peak usage hours to reduce user impact from a migration, and to allow tests that do only a very small number for test purposes. (“Give me a ping, Vasili. One ping only, please.”)


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LGTM :+1:

@martin-brennan can you review and merge if ok?

@johnsonm Looks great, thanks for adding those extra specs and cleaning up the exit codes. Extra points for The Hunt for Red October reference :100:

:tada: thanks both of you for the review!