Add table to track changes to report objects

[preaction]

As we begin to change the reports from what the reporter gave us, we need a way to track these changes. This tracking should give us a way to re-create the original report so as to be able to track down problems with reporter machines.

Create a new table, test_report_changes, with the following schema:

• id - an autoincrement ID to identify the change
• test_report_id - the ID of the test report that was changed

We need to find a good way to express the changes. Some possibilities:

• Two columns, old_value and new_value:
  • old_value and new_value work like JSON masks, both copying the structure of the original object.
  • Array items in old_value missing in new_value means array items were removed.
  • An empty array in old_value and a non-empty array in new_value means array items were added.
  • Object properties in old_value not in new_value were removed.
  • null array items in both old_value and new_value are context, and define what position in the array the value could be found
• One column, changes, describing the changes made to create the new version
  • changes is a JSON array of objects with the following keys:
    • path - The path that was changed
    • old_value - The old value we're replacing, if any
    • new_value - The new value we're adding, if any
    • old_index - Optional, the index of the array changed or removed
    • new_index - Optional, the index of the array changed (moved) or added

We should do some research to find prior art for describing changes to a large object.

Then we should provide a method on the TestReport result class to change the report and add the appropriate history to the changes table.