Fixes to migrate-uploads-data

-removed 'greatest = []', it was being used for testing and should've been removed
-several places should have been using "order_by('date')" not "order_by('-date')"
-should have been trying to select the first object on the exception
-it's taking even less memory than before now, 48MB, have no idea why
-decided to keep "gc.collect()", it makes no difference with chunk_size=5000 but with chunk_size=10000 the memory use climbs, not sure
why that happens only happens with a larger chunk_size