Open one calendar month for an assigned customer

#3516. An assigned accountant (signed in, or the agent service token) opens one whole calendar month, including a month before the customer's first period, so a migration can post source-dated history. The period is created open; close, reopen and the locks are unchanged. Refused: anything but the 1st through the month's last day of a year from 1900 to 2100 (422); a month in a calendar year that holds a filing-locked period, or starting on or before the end of the latest filing-locked period (422, GL_PERIOD_FILED_BOOKS); a month starting before the customer's QuickBooks cutoff (422, GL_PERIOD_BEFORE_QUICKBOOKS_CUTOFF); and a month overlapping a period, including a concurrent create of the same month (409). The audit row records the acting person, or the service key's source and apiKeyId. An unassigned customer is 403.