consider removing the `invoice_id` field from a workorder table

[shoulders]

once you create an invoice from a workoder you can no longer edit a WO so removing the column might makes sense becasue

• I can do a LEFT/RIGHT onto the invoice table to see if there is a match
• updateLastActive() should not have invoice_id in for this reason (on most of the functions
• when an invoices is create the updateLastActive() and writeRecordToActivityLog( are triggered in the invoice insert.
  however
• consider when i do a print or email of a workorde that has an invocie, should i refrence this.
• currently the activity log when i print/email passes the whole record (i.e. all the ids including invoice)