Context:

With OPT-3662 and OPT-3663, we have added new fields to the report configuration, one specifying the custom bucket unit for date variables and one specifying the custom bucket width for number variables (potentially also the x-axis offset). These have default values, but to ensure the field exists for all previously created reports in ES we should add a migration that adds these defaults to existing reports. 

AT:

• Set the groupByDateVariableUnit for all existing to automatic
• Bucket width and baseline can remain `null`, this will result in automatic bucketing for both single and combined reports