Allow customisation of batch size and inter-batch delay

While actually migrating 50,000 users we learned that it is useful to be able to configure the Google API batch size and the delay between calls to the API.

changed milestone to %Sprint 70

added workflowIn Progress label

mentioned in commit 07ec5eeb

mentioned in merge request !2 (merged)

added workflowReview Required label and removed workflowIn Progress label

added small change label

unassigned @rjw57

assigned to @rjw57

added workflowRework label and removed workflowReview Required label

added workflowReview Required label and removed workflowRework label

mentioned in commit 0d289d93

closed via merge request !2 (merged)

closed via commit 07ec5eeb